1 c. butter, softened
2 egg yolks
2 1/4 c. all purpose flour
1/2 tsp. vanilla
2/3 c. sugar
1/2 tsp. salt

Cream butter, gradually add sugar, beating until light and fluffy. Add egg yolks, one at a time, beating well after each. Stir in vanilla. Combine flour and salt; add to creamed mixture, mixing well. Chill. Roll dough into 1 inch balls; place about 2 inches apart on ungreased cookie sheet. Press thumb in each cookie leaving an indentation. Bake at 300 degrees for 20-25 minutes. Do not brown. Cool on wire racks. Place about 1/2 teaspoon frosting in each cookie.

CHOCOLATE FROSTING:

1 c. sugar
1/4 c. butter
1/4 c. cocoa
1/2 tsp. vanilla
1/4 c. milk

Combine sugar, cocoa, and milk in a heavy saucepan. Bring to a boil, and boil 1 1/2 to 2 minutes, stirring constantly. Remove from heat, stir in butter and vanilla. Beat until mixture is of spreading consistency.