Heat oven to 375 In large bowl beat sugar 1 cup butter and the vanilla with electric mixer on medium speed until fluffy Beat in egg yolk until smooth Beat in flour and cocoa
Shape dough by rounded teaspoonfuls into 1 inch balls On ungreased cookie sheet place balls 1 inch apart With inxes finger or thumb make indentation in center of each ball
Bake 7-9 minutes or until set Immediately remove from cookie sheet to wire rack Cool completely about 30 minutes
In small bowl beat 1/4 cup butter and the powdered sugar on low speed until smooth Beat in milk peppermint extract and enough food color for desired color Spoon about 1/2 t filling into each cookie
Melt the chocolate chips and shortening and drizzle over the tops of each cookie