Prepare and bake brownie mix in a greased 13"x 9" baking pan according to package directions. Let cool. Mix powdered sugar, 1/2 cup softened butter and peanut butter. Mix well and spread over cooled brownies. Chill for one hour. Melt together chocolate chips and remaining butter in a saucepan over low heat, stirring occasionally. Spread over brownies. Let cool; cut into squares. Makes 2 to 3 dozen.