Preheat oven to 375F (190C) and line cookie sheets with parchment paper. Set aside.
Combine butter and sugar in large bowl and beat with electric mixer until light and fluffy. Add sour cream, stir well.
Add egg, egg yolk, and vanilla extract. Stir well.
In a separate, medium-sized bowl, whisk together flour, baking powder, baking soda, and salt.
Gradually stir dry ingredients into wet until well-combined.
Roll dough into 1 ½ Tablespoon-sized ball and place on prepared cookie sheet. Use your fingers to gently flatten (if dough sticks to your fingers, wet them first)
Bake on 375F for 6-8 minutes. If cookies puff while cooking, use the clean bottom of a glass to gently re-flatten. Allow cookies to cool while you prepare your frosting.
Frosting
Place butter in a medium-sized bowl and beat until creamy. Gradually add powdered sugar until combined. Stir in vanilla extract and salt.
Add food coloring and stir until even in color.
Use a spatula to spread frosting evenly over cooled cookies. Decorate with sprinkles. Store cookies in airtight container.