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["Chocolate Frosting: Combine sugar, cocoa and milk in heavy saucepan.", "Bring to a boil and boil 1 1/2 to 2 minutes, stirring constantly.", "Remove from heat and stir in butter and vanilla.", "Beat until mixture is of spreading consistency.", "You may want to refrigerate for a little while.", "Cookies: Preheat oven to 300 degrees.", "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 and add to creamed mixture, mixing well.", "Chill until dough holds shape when rolled.", "Roll into 1"" balls and place on ungreased cookie sheet.", "Press thumb in each cookie leaving an indentation.", "Bake at 300 degrees for 20 to 25 minutes.", "Do not brown.", "Cool on wire rack.", "When cool place about 1/2 tsp chocolate frosting in each cookie indentation."]