Preheat oven to 350^. Coat two 12x1 inch round pizza pans with nonstick cooking spray. Press cookie dough evenly into ONE pan. Bake 15 to 20 minutes or until edges are golden brown. Cool 20 minutes in pan on wire rack. Remove from pan; cool completely on wire rack.
Prepare cake mix according to package directions. Fill SECOND pizza pan 1/4 to 1/2 full with cake batter. (reserve remaining batter for another use, such as cupcakes.) Bake 10 to 15 minutes or until toothpick tested done. Cool 15 minutes on wire rack. Gently remove cake from pan; cool completely.
Combine vanilla frosting and peanut butter in small bowl. Gradually stir in milk, 1 Tbsp. at a time, until of spreadable consistency.
Place cookie on serving plate. Spread peanut butter frosting over cookie. Place cake on top of cookie, trimming cookie to match size of cake, if necessary. Frost top & side of cake with chocolate frosting. Garnish with peanut butter cups, if desired.