3 sticks unsalted butter, cut into pieces
12 oz. fine-quality bittersweet chocolate
    (not unsweetened), chopped
6 large eggs
1 1/4 C. cake flour (not self-rising)
1 C. + 2 T. unsweetened cocoa powder
    (not Dutch process)
3 C. granulated sugar
1/2 tsp. salt

Preheat oven to 350°F. Butter and flour a 13 x 9-inch metal baking pan, knocking out excess flour.

Melt butter with chocolate in a large metal bowl set over a saucepan of barely simmering water, stirring until smooth. Remove bowl from pan and whisk in eggs, 1 at a time.

Sift together flour and cocoa powder in a separate bowl and stir into batter with sugar and salt. Pour batter into pan and bake in middle of oven until top is firm and a tester inserted into center comes out with crumbs adhering, 40 to 45 minutes. Cool completely in pan on a rack, about 2 hours, before cutting into giant squares.