In a large mixer bowl beat margarine or butter till softened; beat in 1 1/2 cups powdered sugar and vanilla. Add flour, cocoa powder, cornstarch, and salt; beat till smooth. Stir in chopped peanuts. Reserve 1 1/3 cups of cocoa mixture. Spread remaining cocoa mixture in the bottom and up the sides of an ungreased 10-inch tart pan with a removeable bottom, or in the bottom and 1-inch up the sides of a 10-inch springform pan. In a bowl stir together peanut butter and 1 cup powdered sugar; stir in chocolate pieces. Carefully spread peanut butter mixture over crust. Spoon the remaining cocoa mixture over peanut butter mixture; carefully spread to cover. Using tines of a fork, score shortbread into 16 wedges. Bake in 325 degree oven about 50 minutes or till surface looks slightle dry. Cool slightly. While warm, place a chocolate kiss on each wedge. When kiss softens (about 10 minutes), top with a peanut. Score wedges again. Cool completely. Remove sides of pan; cut into wedges.