Childhood Thumbprint Cookies

Childhood Thumbprint Cookies
This recipe came from Southern Living magazine, December 1985.

1 c. butter
2/3 c. sugar
2 egg yolks
1/2 t. vanilla extract
2 1/4 c. flour
1/2 t. salt
Cream butter; gradually add sugar, beating until light and fluffy. Add egg yolks, one at a time, beating well after each addition. Stir in vanilla.
Combine flour and salt; add to creamed mixture, mixing well. Chill.
Roll dough into 1" balls; place balls about 2" apart on ungreased cookie sheets.
Press thumb in each ball of dough, leaving an indentation. Bake at 300 degrees for 20 to 25 minutes; do not brown. Allow to cool on wire racks. Place about 1/2 t. chocolate frosting in each cookie indentation.
Chocolate Frosting:
1 c. sugar
1/4 c. cocoa
1/4 c. milk
1/4 c. butter
1/2 t. vanilla extract
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.