Heat oven to 375. Coat an 8 in square pan (or similar pan) with 1 tbsp. butter. Melt chocolate with remaining 7 tbsps butter (in double boiler so it does not scald). Meanwhile, beat eggs in medium mixing bowl at high speed 1 minute. Gradually add sugar to eggs, beating constantly. Continue beating until mixture is pale yellow and reaches consistancy of soft-peaked whipped cream, 4-5 mins. Reduce mixer speed to medium; beat chocolate mixture and vanilla into egg mixture until thoroughly blended. Add flour to chocolate mixture; beat at low speed just until flour is absorbed. Stir in nuts if desired using a spoon. Spoon batter into prepared pan. Bake until cake tester or wooden toothpick comes out clean, about 30 mins. Cool completely (do not refrigerate). Cut into 16 squares. Dust with powdered sugar if desired just before serving. For extra-chewy texture, cover brownies and let stand at room temperature overnight.