3 cups chunky peanut butter
1 1/2 cups butter or margarine, melted and divided
4-1/2 cups powdered sugar, sifted
1 cup brown sugar, firmly packed
1 teaspoon vanilla extract
2 cups semisweet chocolate chips
Combine peanut butter, 1 cup melted butter, powdered sugar, brown sugar and vanilla in a large bowl; mix well.
Spread on a jelly-roll pan (15 x 10 x 1-inch) and press down evenly.
Melt chocolate chips and remaining butter in a small saucepan over low heat, about 6 minutes. Remove from heat and spread evenly over the peanut butter dough.
Refrigerate until firm, about 1 hour, before cutting into squares.
Makes 24 squares.