Instructions: To make cookies: Grease cookie sheets. Beat butter, shortening, and peanut butter at medium speed. Gradually add powdered sugar, beating until smooth. Add egg yolk and vanilla. Add flour, mixing well. Cover dough and refrigerate 1 hour. Prepare frosting.

Shape dough into 1" balls. Gently flatten balls with finger tips. Bake at 350 for 16 to 18 minutes. Remove cookies from sheets to cool completely. Spread about 1 teaspoon frosting evenly over top of each cookie. Sprinkle with ground peanuts (optional).

To make frosting: Beat butter and peanut butter until smooth. Gradually add powdered sugar and vanilla until blended, but crumbly. Add milk, 1 tablespoon at a time, until smooth. Refrigerate until ready to use.

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