For the crust:: The warm, buttery crumbs should smell toasty and bind together when pressed, which tells you the texture is correct. If the crumbs feel dry, they will not hold their shape in the muffin cup, so check that the melted butter is fully incorporated. You want to press firmly so the crust forms an even layer, and a common mistake is pressing too lightly, which can cause the crust to fall away when serving.
Preheat oven to 325 degrees. In a mixing bowl, using a fork, stir together graham cracker crumbs and sugar, then pour in melted butter and stir until evenly moistened.: You should notice the oven reaching temperature steadily, and a consistent heat helps the cheesecakes bake evenly. Preheating ensures the center sets at the same rate as the edges; skipping preheat can lead to uneven rise. Avoid opening the oven repeatedly during this stage, which drops the temperature.
Add 1 slightly heaping tablespoonful to 12 paper lined muffin cups. Press crust firmly into an even layer. Bake in preheated oven 5 minutes then remove from oven and allow to cool.: The mixture should look uniformly damp, with no dry pockets of crumbs. When you stir, you should hear a soft scraping as the crumbs clump. If any dry bits remain, the crust will be crumbly after baking. A common error is adding cold butter which can re solidify and leave clumps, so ensure the butter is warm and fluid.
For the raspberry swirl:: As you press, the crumbs will compact and the surface will appear smooth and even. I press with the back of a spoon or a small measuring cup to get a uniform thickness. If you press unevenly you may have thin spots that burn or thick spots that remain soft.
Add raspberries and 2 tbsp granulated sugar to a food processor and pulse until well pureed, about 30 seconds - 1 minute. Press mixture through a fine mesh strainer into a bowl (basically until there are only seeds remaining in strainer, you should get quite a bit out of it). Set aside.: The crust should smell nutty and become slightly firmer to the touch after this quick bake. Cooling prevents the crust from becoming soggy when you add the filling. Removing them too soon risks a fragile crust, and overbaking will make the crust too hard.
For the cheesecake filling:: The puree will smell vividly fruity and bright, and pressing it will leave behind seeds in the strainer while a silky sauce collects below. Use the back of a spoon to press as much through as possible to get a smooth texture. A common mistake is skipping the straining, which leaves seeds in the swirl and changes the mouthfeel.
In a mixing bowl, whisk together granulated sugar and flour. Add cream cheese and lemon zest using an electric hand mixer, blend mixture just until smooth. Mix in eggs one at a time.: Letting the raspberry sauce rest slightly allows the flavors to meld and any excess liquid to settle. This keeps the swirl from bleeding too aggressively into the filling. If you rush to use it while steaming hot, the heat can affect the filling texture and cause more bleeding.
Stir in vanilla and sour cream just until combined. Tap bowl forcefully against countertop, about 10 times, to release large air bubbles. Divide mixture evenly among cups over crust layer, adding about 1/3 cup to each and filling nearly full.: When combined, these dry ingredients will feel even and powdery, which helps them disperse evenly through the filling. This step prevents pockets of flour or sugar that might not integrate well. Not mixing dry ingredients first can lead to grainy spots in the batter.
Jiggle pan to level cheesecake filling then dollop about 5 small circles of raspberry sauce over each cupcake, about 3/4 tsp total over each one. Using a toothpick, swirl raspberry filling with cheesecake mixture to create a marbled design.: The bowl will transform from lumps to a glossy, velvety mass, and the lemon oil will lift the aroma. Blend gently to avoid overheating the cream cheese . Overmixing can incorporate too much air and lead to cracks or a grainy texture.
Bake in preheated oven 22 - 25 minutes until cupcakes are puffed and nearly set (they might crack a little but they will deflate while cooling and you won't even see it).: After each addition, the batter should look more unified and slightly thicker. Adding eggs slowly helps them incorporate without curdling. A typical mistake is dumping both eggs in at once, which can cause uneven texture and make the batter split.
Remove from oven and allow to cool completely, then chill in refrigerator 3 hours, until set. Store in refrigerator in an airtight container.: The batter will become silkier and slightly looser, with a subtle tang from the sour cream . Combining briefly preserves a dense, creamy texture. Overworking at this stage again risks creating too much air, which can alter the rise and lead to cracking.
Tap bowl forcefully against countertop, about 10 times, to release large air bubbles: You will see big bubbles rise and pop, leaving a smoother surface, which helps prevent large holes in the baked cheesecakes. This quick action is more effective than a long slow stir at removing trapped air. Forgetting to do this can leave visible air holes and an uneven texture.
Divide mixture evenly among cups over crust layer, adding about 1/3 cup to each and filling nearly full: The batter should settle smoothly into each cup, and the surface will look glossy and level once poured. Using the same scoop for each cup ensures even baking. Overfilling will cause spillover and uneven doneness.
Jiggle pan to level cheesecake filling then dollop about 5 small circles of raspberry sauce over each cupcake, about 3/4 tsp total over each one: The gentle jiggle helps remove small pockets of air and levels the batter, while the small dollops of sauce create a marbling opportunity. The sauce should sit on top without sinking immediately. If the sauce sinks too much, it was likely too thin or the batter too warm.
Using a toothpick, swirl raspberry filling with cheesecake mixture to create a marbled design: The toothpick will drag ribbons of color into soft, pretty patterns, and the motion should be delicate to avoid over mixing. You want distinct streaks, not a blended pink. Over swirling will lose the marbled effect and mix too much raspberry into the batter.
Bake in preheated oven 22 to 25 minutes until cupcakes are puffed and nearly set: You will notice the edges firm and the centers slightly jiggly but not liquid. The smell becomes richer and pleasantly sweet. Slight cracking can occur, but the cheesecakes will settle as they cool. Opening the oven too often during this stage can cause temperature swings and uneven baking.
Remove from oven and allow to cool completely, then chill in refrigerator 3 hours, until set: Cooling slowly prevents shock that can make cracks more severe, and chilling firms the texture for clean slices. The texture changes from warm and pillowy to a silky, firm mouthfeel. Rushing to unwrap them while still warm can cause them to collapse.
Store in refrigerator in an airtight container: Proper storage preserves texture and freshness, and these mini cheesecakes keep well for several days chilled. An airtight container prevents drying out and protects against fridge odors. Leaving them uncovered will dry the tops and dull the flavor.