PREHEAT oven to 325° F. Grease 8-inch-square baking pan.
MELT margarine or butter and baking bar in small, heavy-duty saucepan over low heat, stirring until smooth. Remove from heat; cool until lukewarm.
COMBINE cake meal and cocoa in small bowl. Beat sugar and eggs in large mixer bowl until mixture is thick and light in color. Stir in sour cream and melted chocolate mixture. Add cocoa mixture and beat at low speed until just combined. Stir in nuts, if desired. Spread batter into prepared pan.
BAKE for 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool completely in pan on wire rack. Cut into squares.
NOTE: These brownies will keep for several days in a covered container.