Preheat oven to 350 degrees F.
Blend melted butter with granulated sugar and brown sugar in bowl of large mixer. Add vanilla and eggs and blend well on slow speed until combined.
Stir together cocoa, flour, baking soda, and salt in separate bowl. Fold into batter and blend well, scraping bottom of mixing bow to ensure ingredients are evenly combined, 2 to 3 minutes. Fold in nuts if using.
Pour or spoon into 13x9 inch baking pan greased with cooking spray, and lined with parchment paper. Place pan on baking sheet. Bake at 350* until brownies are set ,and slightly firm but not dried out, about 40 to 45 minutes.
Cool, then freeze 1 hour before cutting. To cut, unmold from baking pan. Peel of parchment paper. Cut into large blocks (about 2 1/2x3inches for 513 calorie version!) or smaller bars
Wrap each in wax paper and keep refrigerated, bring to room temperature before serving.
May be frozen for up to 1 month.
Makes 15-24 brownies depending on how large or small you cut them.