Mix butter, sugar, egg yolks, vanilla, and sour cream together. Add dry ingredients to mixture. Wrap in wax paper and refrigerate overnight. Roll out on a lightly floured surface or pastry cloth. Cut into 1/8th to 1/4th inch thickness using your favorite cookie cutters. Bake at 350°F for approximately 10 minutes or until the bottom is lightly browned. May need more time if on the thicker side. Cool on wire rack. Store in airtight container in the refrigerator until ready to apply icing.
Cream butter, cream cheese, and vanilla together. Gradually add powder sugar until blended. Thin with milk to desired consistency. If adding coloring paste, you may not need milk to thin.
After icing cookies, lay flat in the fridge until set. Then store in a airtight container with a small square of wax paper between cookies. Store in fridge until ready to serve.