Preheat oven to 350 degrees (F). Line a 12 mold cupcake tin with paper liners and set aside.: You should smell a faint warm air once the oven begins to come up to temperature, and visually the oven indicator will show the set heat. Preheating ensures the cupcakes rise immediately and bake evenly, creating a domed top rather than a flat one. A common mistake is putting batter into a cold oven which leads to poor rise and dense centers, so wait until the oven fully reaches 350 degrees F . If your oven runs hot, use an oven thermometer to avoid overbaking.
In a medium bowl combine the flour, baking powder, baking soda, and salt; whisk well and set aside.: You will see the dry mixture lighten as the baking powder and baking soda disperse. Whisking aerates the flour slightly and ensures the leaveners are evenly distributed, preventing uneven rise. The mixture should look uniform with no visible clumps of leavening. Skip thorough whisking and you risk pockets of bitterness or uneven texture in the cupcakes.
In a large bowl using a handheld electric mixer, or in the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and creamy, about 30 seconds. Add in the sugar, lemon extract and lemon zest and beat until well combined. Add in the egg and egg yolk and mix well, scraping down the sides of the bowl as needed. Reduce the mixer speed to low. Add in 1/2 of the flour mixture and mix until just combined (this should take about 5 seconds). Add 1/2 of the sour cream and mix until just combined. Repeat with remaining flour and sour cream, being sure to mix on low until just combined. Turn mixer off. Using a rubber spatula, quickly fold in the lemon juice.: You will notice the unsalted butter change from matte and slightly stiff to glossy and softer, and the mixer makes a steady whipping sound. This creaming step traps air which helps with lift, so don’t rush it. A typical pitfall is overbeating at high speed which can break down the butter, making the batter greasy and the crumb heavy. Keep speed moderate for best aeration.
Divide the batter evenly among the prepared muffin cups, filling them 3/4 of the way full. Bake for 16 to 18 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. Once cupcakes are cool, frost and decorate. The cupcakes may also be frozen, unfrosted, and stored in a freezer safe bag, for up to 2 months.: The bowl will become fragrant with citrus oils from the lemon zest and the texture will turn paler as sugar dissolves into the butter. This step builds flavor and contributes to a light crumb. If the sugar remains grainy, continue beating briefly until the mixture looks smooth, but beware of beating so long that the butter becomes too soft.
In a small saucepan combine blueberries and lemon juice; bring to a simmer over medium heat and cook, stirring constantly and gently smashing the blueberries smooth, until thickened into a deep, dark blueberry reduction, about 10 minutes. Remove from heat. Place a fine-mesh strainer over a small bowl and push the puree though, leaving any skin/large chunks behind. Allow mixture to cool completely before using. If you'd like to have enough blueberry puree to drizzle on the cupcakes, double this recipe and set half aside for decorating.: You will hear a slight change in the mixer tone as the egg incorporates, and the batter becomes more cohesive. Eggs add structure and richness. Scraping the bowl prevents streaks of unmixed butter or flour. A common error is adding cold eggs which can cause the mixture to seize; use room temperature eggs for smooth integration.
Once the blueberry puree has cooled, place the softened butter in a large bowl or in the body of a stand mixer. Using a handheld mixer, or with the paddle attachment on your stand mixer, beat butter on medium-high (scraping down the sides as needed) until completely smooth; about 2 minutes.: The batter should look slightly streaky but mostly unified, with no large pockets of dry flour . Mixing briefly preserves tenderness by limiting gluten development. Overmixing here will produce a chewier, denser cupcake, so mix only until you no longer see visible dry flour.
Add in a 1/4 cup of the blueberry puree and beat until completely combined; about 1 minute (don't worry if the mixture looks weird at this point).: The batter will become looser and creamier as the full fat sour cream folds in. This ingredient adds moisture and subtle tang. If you see a broken or curdled texture, stop mixing and gently fold with a spatula until the batter smooths out, because heavy mechanical mixing can split the emulsion.
Add in the confectioners sugar, one cup at a time, on low speed. Once all of the sugar has been added, add in the cream and salt, then beat on high-speed for a full 2 minutes - this thickens up the frosting and makes it nice and fluffy.: After the second addition the batter should be smooth and slightly thick, with a silky sheen. The gentle mixing keeps the crumb delicate. If the batter becomes overly elastic or shiny, you may have overworked it and risk a tougher cupcake, so proceed with care.
Frost cooled cupcakes as desired.: Folding in lemon juice at the end preserves the fresh citrus lift and reacts with the baking soda to create a final lift. You should see tiny flecks of zest suspended in the batter and smell a bright citrus note. Avoid vigorous stirring which can deflate the air you just built; gentle folding is key.
Divide the batter evenly among the prepared muffin cups, filling them 3/4 of the way full. Bake for 16 to 18 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. Once cupcakes are cool, frost and decorate. The cupcakes may also be frozen, unfrosted, and stored in a freezer safe bag, for up to 2 months.: You will see the tops turn a pale golden color and spring back slightly when touched. The toothpick test should show few crumbs, not wet batter. Cooling in the pan initially prevents the fragile cakes from collapsing, while finishing on a wire rack keeps the bottoms from steaming. A frequent mistake is frosting warm cupcakes which melts the frosting; always wait until they are fully cool before frosting.
In a small saucepan combine blueberries and lemon juice; bring to a simmer over medium heat and cook, stirring constantly and gently smashing the blueberries smooth, until thickened into a deep, dark blueberry reduction, about 10 minutes. Remove from heat. Place a fine mesh strainer over a small bowl and push the puree through, leaving any skin/large chunks behind. Allow mixture to cool completely before using. If you'd like to have enough blueberry puree to drizzle on the cupcakes, double this recipe and set half aside for decorating.: As the blueberries simmer you will hear a gentle bubbling and smell a concentrated fruity aroma; the mixture will change from bright purple to a deep jewel tone as water evaporates. Straining creates a silky puree free of seeds and skins, which is essential for smooth buttercream. If you skip straining, the frosting can have grainy bits and uneven color.
Once the blueberry puree has cooled, place the softened butter in a large bowl or in the body of a stand mixer. Using a handheld mixer, or with the paddle attachment on your stand mixer, beat butter on medium high scraping down the sides as needed until completely smooth about 2 minutes.: The butter will look paler and fluffier, and the beaters will make a steady whoosh. Properly aerated butter creates a light frosting base. If the butter is too cold the texture will be lumpy, and if melted it will be greasy, so aim for very soft butter for best results.
Add in a 1/4 cup of the blueberry puree and beat until completely combined about 1 minute dont worry if the mixture looks weird at this point.: The mixture may first look streaky or slightly separated as the fruit introduces moisture, but continued beating incorporates the puree into a smooth colored base. This step infuses real fruit flavor into the frosting. If the mixture appears watery, scrape down and beat longer; if it remains too loose, chill briefly and then rewhip.
Add in the confectioners sugar, one cup at a time, on low speed. Once all of the sugar has been added, add in the cream and salt, then beat on high speed for a full 2 minutes.: As you add confectioners sugar the frosting will thicken and become opaque, and beating at the end makes it light and fluffy. You should hear a change to a lighter, higher pitched sound from the mixer and see peaks form as the frosting thickens. Adding sugar too quickly can create a gritty texture, so add it gradually and keep the speed low until fully incorporated.
Frost cooled cupcakes as desired.: When you apply the frosting it should hold its shape and pipe smoothly, with a glossy finish and a pleasant, balanced sweetness. The aroma of cooked blueberries and butter will be prominent. If the frosting seems too soft, chill briefly and then pipe; if too stiff, add a teaspoon of heavy cream at a time until desired consistency is reached. Present cupcakes on a flat platter and add a few fresh blueberries if using, just before serving to avoid moisture transfer.