PREHEAT oven to 350º F.
COMBINE flour and baking soda in small bowl. Beat brown sugar, peanut butter, butter and granulated sugar in large mixer bowl until creamy. Beat in egg and vanilla extract. Gradually beat in flour mixture. Stir in chunks. Press into ungreased 13 x 9-inch baking pan, distributing chunks evenly.
BAKE for 18 to 22 minutes or until center is set. Cool completely in pan on wire rack. Cut into bars.