Peanut Butter and Chocolate Cookie Sandwiches

1/2 cup butter, softened
1/2 cup sugar
1/2 cup brown sugar
1/2 tsp baking soda
1/2 tsp salt
1 egg
1 tsp vanilla
1 cup all purpose flour
1/2 cup unsweetened cocoa powder
1 (11 oz) pkg peanut butter and chocolate pieces
1/2 cup chocolate frosting
1/2 cup peanut butter

Beat butter for 30 seconds.  Add sugar, brown sugar, soda, and salt.  Beat until combined.  Add egg and vanilla.  Beat till combined.  Beat in flour and cocoa powder.  Stir in peanut butter and chocolate pieces.  Cover and chill for 2-3 hours. 

Preheat oven to 375.  Roll dough into 1 inch balls.  Place 2 inches apart on lightly greased sheets.  Flatten slightly.  Bake 7-8 minutes.  In a small bowl, stir together frosting and peanut butter.  Spread 2 tsp frosting mix on bottom half of cookies.  Top with another cookie.