Step 1: One 13×9×2-inch pan, buttered and lined with buttered parchment or foil. Set the rack in the middle of the oven and preheat to 350 degrees F.
Step 2: Bring a saucepan of water to a boil and turn off heat. Combine butter and chocolate in a heat proof bowl and set over pan of water. Stir occasionally until melted.
Step 3: Whisk eggs together in a large bowl, then whisk in the salt, sugars, and vanilla. Stir in the chocolate and butter mixture, then fold in the flour.
Step 4: Pour batter into prepared pan and spread evenly. Bake for about 45 minutes, until top has formed a shiny crust and batter is moderately firm. Cool in pan on a rack.
Step 5: Wrap pan in plastic wrap and keep at room temperature or refrigerated until the next day.
Step 6: To cut brownies, unmold onto 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.