PREHEAT oven to 350°F. Beat sugar, peanut butter and butter in large bowl with electric mixer on medium speed until well blended. Add eggs and vanilla; mix well. Add flour and salt; beat until well blended. Stir in chocolate.
LINE 9-inch square baking pan with foil, with ends of foil extending over sides of pan. Grease foil. Spread batter into prepared pan. Drop jam by spoonfuls over batter; swirl with knife to marbleize.
BAKE 35 to 40 minutes or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan. Lift dessert from pan, using foil handles. Cut into 24 squares to serve.