- 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 a pick inserted in center comes out with crumbs, 25 to 35 minutes. Cool completely.