Preheat oven to 350°. Stir together butter and chocolate in a medium-size heavy saucepan over low heat, stirring often, 3 to 5 minutes or until chocolate is melted. Remove from heat, and stir in breadcrumbs and sugar until well blended. Press mixture onto bottom of a lightly greased 9-inch square pan.
Bake at 350° for 8 minutes. Cool on a wire rack 15 minutes. Chill 30 minutes.