In a mixing bowl, cream the butter and sugar. Beat in eggs. Add sour cream and vanilla, and mix well.
Combine flour, baking powder and baking soda, add to the creamed mixture and mix well. Chill dough at least 2 hours or overnight.
Roll dough out on a heavily floured surface to a 1/4 inch thickness. Cut with a 3-inch cookie cutter or use a drinking glass.
Place cookies on a lightly greased baking sheet. Bake at 350 degrees F for 8 to 12 minutes, or until cookies springs back when lightly touched. Cool.
Mix well - powdered sugar, crisco, salt, and milk.
Add melted butter and vanilla. Blend thoroughly, and spread on cookies. Decorate with sprinkles or colored sugar if desired.