Line baking sheets with Silpat or parchment paper.
Whisk together dry ingredients and set aside.
MAKE BATTER
Mix together butter, sugar, and powdered sugar until light and fluffy.
Add egg, egg yolk, sour cream, vanilla and mix until combined.
Add in dry ingredients and mix well.
Split dough into 2 sections. Roll out dough to 1/4" or 3/8" between 2 pieces of parchment paper cut to the size of your baking sheets.
Chill dough for 1 hour to overnight.
EGG PAINT
Place egg yolk and water into a bowl. Mix together and separate out into desired number of bowls to color.
Add a little bit of color into each bowl.
Paint cookies and then bake.
BAKE
Preheat oven to 375°. Remove the top sheet of parchment and use it to line your baking sheets. Cut the chilled dough into shapes, chill again for 5-10 minutes and bake for 10-12 minutes.
If your going to re-roll any scraps, you will need to follow the same process: re-roll scraps between parchment paper, chill for at least 1 hour, cut shapes and chill once cut for 5-10 minutes before baking.