Chocolate Brownies

Preheat oven to 350°F. Butter and flour a 13- by 9- by 2-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 20 squares.