Martha Stewart also has a really good recipe called "Peanut Butter Whoopie Pies" as well. The recipes are similar but sometimes one turns out better than another.
2 c. all-purpose flour
1/2 c. unsweetened cocoa powder
1/2 c. hot water
1 t. vanilla extract
1 t. baking soda
1 c. white sugar
1 c. butter
1 T. butter
1 3/4 c. powdered sugar
1/4 c. peanut butter
1//2 c. milk
1. Cream together 1 c. of butter & the egg. ADd 1 c. white sugar, vanilla & hot water. Stir in flour, cocoa & baking soda & mix well.
2. Drop from teaspoon onto cookie sheets and bake at 375* for 10-12 minutes. Sandwich cookies together with frosting.
3. Frosting: Blend 1 T. butter & peanut butter. Stir in powdered sugar & add enough milk to make frosting soft enough to spread. Spread between flat sides of cookies! Enjoy!