• 7 ounces bittersweet chocolate , coarsely chopped
• 1/2 cup unsalted butter , cut up
• 1 cup sugar
• 2 tablespoons flour , plus more for pan
• 4 eggs , separated

Preheat oven to 325°. Butter and flour a 9x13-inch baking pan. In a microwave-proof bowl, combine chocolate and butter. Microwave on medium (50% power) for 2 1/2 minutes, stirring once halfway through. Stir until chocolate melts.

Whisk sugar and flour into chocolate mixture, then whisk in egg yolks. In a large mixing bowl, beat egg whites until stiff. Fold beaten whites into chocolate mixture until just blended. Pour batter into prepared pan. Bake 30 minutes or until a cake tester inserted into center comes out clean.

Cool completely and cut into squares.