Put sugar in mixing bowl.
Melt chocolate and butter together over low heat, stir until well blended and smooth. Add to sugar; mix well. Add beaten egg yolks; mix well. Add flour and vanilla; mix well. Stir in chopped pecans. Fold in beaten egg whites.
Spread in a 10x15-1-inch greased and floured jelly roll pan. Bake at 325° for 20 to 25 minutes.
Melt butter and chocolate over low heat; stir until smooth then add confectioners' sugar. Heat milk until hot; add to chocolate mixture. Add vanilla; beat until smooth. Spread over cooled brownies and sprinkle with chopped pecans, if desired.