1. Use an electric mixer to beat together butter and sugar . 2. Add egg and vanilla extract and beat some more. 3. Mix together flour, baking powder and salt. 4. Add dry ingredients to wet ingredients and mix until cookie dough forms. 5. Divide cookie dough in 5 parts. Color each one with separate food coloring. Knead until each cookie dough portion is completely tinted. 6. Roll all cookie dough colors into a large sphere.  7. Wrap cookie dough in plastic and refrigerate 1 hour. 8. Remove the cookie dough from refrigerator. 9. Preheat oven to 375 degrees F. 10. Add flour to a clean working surface. 11. Roll out cookie dough so that it’s about ¼ of an inch. 12. Using a circular cookie cutter, about 2 ½ inches in diameter, cut out circles of your cookie dough.  13. Bake for 7-9 minutes. 14. Beat together butter, sugar, whole milk and vanilla extract. 15. Once buttermilk is fully mixed, scoop into an icing bag with Wilton tip #10 attached. 16. Flip over half of your cookies and add frosting. Sandwich with another cookie.