Preheat the oven to 350° and line 2-3 cookie sheets with parchment paper.: The kitchen will fill with a warm, faint baking scent as the oven comes up to temperature, and having lined pans prevents sticking while keeping the bottoms pale and even. Ensuring the oven is fully up to temperature matters because an oven that is too cool will cause spreading, and one that is too hot will brown edges before centers set. A common mistake is not rotating racks between batches, which can yield uneven color; rotate halfway through baking when your recipe calls for it. Use an oven thermometer if you suspect your oven runs hot or cold, and space the pans so air circulates around each cookie sheet.
In a medium bowl whisk together the flour, salt, and baking powder. Set aside.: When you whisk, you will notice the dry ingredients become homogenous, which prevents pockets of leavening or salt. The dry mix should look uniform and light. Mixing these first ensures they are evenly distributed once added to the wet ingredients, and that uniformity helps every cookie rise the same. Avoid overmixing at this stage, which can activate gluten unnecessarily. If your flour seems clumpy, sift it or break lumps with the back of a spoon to avoid dense spots in the dough.
In another medium bowl, beat together the butter and sugar for 2-3 minutes until light and fluffy. Add the egg yolks, sour cream, and extracts and mix until just combined.: As you beat the Unsalted Butter and Granulated Sugar , you are incorporating air, which is why the mixture changes to a paler color and takes on a slightly fluffy texture. Adding the Egg Yolks , Sour Cream , and extracts afterward enriches the mixture, and you will smell the warmth of the vanilla and lemon as they release their aromas. Mixing until just combined keeps the dough tender, but overbeating after adding yolks can introduce too much liquid incorporation, leading to spreading. A common error is using melted butter, which prevents proper aeration; keep butter softened at room temperature instead.
Add the dry ingredients to the wet and mix until just barely combined.: At this stage, the dough will look slightly shaggy and come together in a mass. Mixing until barely combined preserves tenderness, because overworking develops gluten and yields tougher cookies. You want enough mixing that no streaks of flour remain, but stop once the dough holds together. If you see a very sticky dough, a short rest of five minutes helps hydrate the flour and make rolling easier. Be careful not to add extra flour to correct stickiness, or you risk drying out the dough.
This recipe does not require any chilling time, so you’re ready to roll out your cookies! Split the dough into 2-3 pieces and roll them one at a time on a lightly floured surface until they are 1/4 inch thick. Use your favorite Christmas tree cookie cutters (or alternative shapes!) to cut out your cookies. Press the scraps together and set them aside to rest for about 5 minutes. Reroll the scraps and cut more cookies until all the dough has been used.: Rolling gives you tactile feedback when dough is the right thickness, you will feel uniform resistance under the pin and see an even surface. Working in batches prevents the remaining dough from warming too much from your hands, which can make it sticky. Resting scraps briefly keeps gluten relaxed so rerolled cookies do not become tough. A mistake here is rolling too thin which yields brittle cookies, so aim for that 1 1/4 centimeter thickness. Keep extra flour minimal, dusting lightly, as excess will dry the dough.
Place the cookies on the parchment-lined cookie sheets leaving 1 1/2 inches between them. Bake for 6-8 minutes, rotating the pan at 4 minutes. I find this gives the cookies a nice, even bake. The edges will be just set and the cookies will be very light in color and should have very minimal coloring underneath them.: As cookies bake you will notice a faint, sweet aroma and the tops will lose their wet sheen, signaling doneness. The described timing produces cookies that are set but not browned, preserving that delicate look. Rotating the pan evens out hot spots in the oven and keeps color consistent. Avoid overbaking, which creates crisp, dry cookies instead of tender ones; check at the lower end of the time range if unsure. If bottoms brown too quickly, lower the rack position or reduce oven temperature by 10 degrees.
Let the cookies cool on the pan for 2 minutes for the shape to set before transferring them to a cooling rack. Frost, once they are cooled then store in an airtight container until you are ready to frost them.: Cooling on the pan lets the cookie set so edges do not distort when moved, and transferring to a rack prevents steam from making them soggy. You will feel the cookie firm slightly as it cools; that is your cue to move it. Storing cooled cookies airtight preserves tenderness until you are ready to ice. A common misstep is icing while still warm, which melts the outline and floods and ruins detail, so be patient and allow complete cooling.
Using an electric mixer with the whisk attachment, mix the powdered sugar and meringue powder together on low.: Mixing these on low prevents a dust cloud and helps the powders begin to hydrate evenly. You will notice the dry mix take on a faint sheen as initial moisture integrates. Proper mixing here prevents lumps and gives a stable base for the royal style icing. Avoid adding water too early or mixing on too high a speed at this stage, which can incorporate too much air and make consistency difficult to control.
Add 8 tablespoons of water and mix together on high speed for about 2 minutes. The icing should be pretty stiff. Add additional water 1 tablespoon at a time until the icing is about the consistency of soft-serve ice cream and forms soft peaks. Reserve 1 cup of frosting and cover tightly with plastic wrap.: At high speed the mixture will thicken and become glossy; this signals the meringue powder activating and the icing stabilizing. The soft-serve consistency indicates a stiff piping icing that will hold shape when outlining. Reserving one cup of this thicker icing gives you the border material that prevents floods from seeping. A frequent error is not sealing the reserved portion, which can form a crust; press plastic wrap directly on the surface.
With the remaining icing, add additional water 1-2 teaspoons at a time until when the whisk is raised the icing drips back into the bowl and the lines of icing take about 10 seconds to melt back into the icing. This is the thin icing that will be used to fill, or flood, your cookies. Separate into small bowls and tint the icing with your chosen colors. Cover each bowl tightly with plastic wrap when not being used. This icing will dry very quickly.: The thin flood consistency should flow smoothly like honey from a spoon and settle flat on the cookie surface. Tinted bowls allow you to work quickly with a palette of colors. Because thin icing dries fast, work over a damp towel or small tray to avoid crusting on the surface. A mistake is adding too much water too quickly, making the icing runny and unable to hold any texture; add water slowly and test often.
Separate the reserved thicker icing into bowls and tint the bowls the colors you want to make your eggs. This will be the frosting that is piped around the outside of the cookie to hold the thin icing in the center. Be sure to cover the bowls until ready to use. When ready to ice the cookies, pipe the thicker outline and then flood the center with the thin icing and add the details with thin icing.: Piping the thicker outline first creates a dam that keeps the flood icing neatly contained, and you can watch the flood level rise to the edge until it meets the piped border. The contrast between the matte outline and glossy flood is satisfying to watch take shape. Add details while the flood is still tacky for best adhesion. Avoid overworking the surface with the tip, which can create marks rather than smooth finishes; use gentle motions and let the icing settle on its own.