To Prepare
Butter and flour a 9x13-inch baking pan. Line the bottom with foil; butter and flour the foil, shaking off the excess
Melt the unsweetened chocolate in a double boiler over simmering water, stirring until smooth. Set aside.
Cream the butter in a mixer bowl until light. Add the sugar and brown sugar, beating until fluffy. Add the melted chocolate, mixing at low speed.