Preheat the oven to 350° and line 2-3 cookie sheets with parchment paper.
In a medium bowl whisk together the flour, salt, and baking powder. Set aside.
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.
Add the dry ingredients to the wet and mix until just barely combined.
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 ¼ 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.
Place the cookies on the parchment-lined cookie sheets leaving 1 ½ 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.
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.
Using an electric mixer with the whisk attachment, mix the powdered sugar and meringue powder together on low.
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.
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.
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..