One 12 x 18-inch jelly roll pan (commercial half-sheet pan) buttered and lined with buttered parchment or foil.
Preheat oven to 350 degrees and set a rack in the middle level. Prepare the pan and set aside. Bring a saucepan of water to a boil and turn off heat. Combine butter and chocolate in a heatproof bowl and set over pan of water, stirring occasionally until melted. Whisk eggs in a large bowl and whisk in salt, sugar, brown sugar and vanilla. Stir in chocolate and butter mixture, then fold in flour. Pour batter into prepared pan and spread evenly. Bake for about 1 hour, until top has formed a shiny crust and batter is moderately firm. Cool in pan on a rack. Wrap pan in plastic wrap and keep at room temperature or refrigerated until the next day.
To cut brownies, unmold to a cutting board, remove paper and replace with another cutting board. Turn cake right side up and trim away edges. Cut brownies into 2-inch squares. Wrap individually in plastic wrap and place in layers between parchment or waxed paper in a tin or plastic container that has a tight-fitting cover. Store at room temperature or freeze.
Variations: Add 4 cups (1 pound) walnut or pecan pieces to the batter.