This is Epicurious’ recipe, unaltered. They make a very chewy, dense brownie, and with the size of the pan, rather thin.
I believe there’s no point to owning a double-boiler. I use a mixing bowl over one of my soup tureens.
2 sticks (1/2 pound) unsalted butter
8 ounces fine-quality bittersweet chocolate (no more than 60% cacao if marked)
2 cups sugar
1 teaspoon pure vanilla extract
5 large eggs
2/3 cup all-purpose flour
1/3 cup unsweetened Dutch-process cocoa powder
1/2 teaspoon salt
Preheat oven to 350°F with rack in middle. Butter and flour a 13- by 9-inch baking pan.
Melt butter and chocolate in a 3-qt heavy saucepan over low heat, stirring, until smooth. Remove from heat and cool to lukewarm. Whisk in sugar and vanilla. Whisk in eggs 1 at a time until mixture is glossy and smooth.
Whisk together flour, cocoa powder, and salt, then whisk into chocolate mixture until combined. Spread in pan and bake until middle looks just wet – before the edges are crispy – about 25 to 35 minutes.