In a large bowl, combine the brown sugar, powdered sugar, butter, and peanut butter, and beat them with an electric mixer until they are smooth and well blended. Pat into an ungreased 15 ½ x 10 ½ x 1-inch pan. Roll the mixture flat on top with a rolling pin.

Melt the chocolate chips and butter on top of a double boiler or in the microwave. Spread the chocolate mixture over the peanut butter mixture and cut into squares while the chocolate is still warm and soft.

Chill the mixture for about 15 minutes and remove the squares from the pan.