• 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 1/2 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.