8½ ounces (2 sticks plus 1 tablespoon) unsalted butter, more for greasing pan
7 ounces bittersweet chocolate
5 large eggs, separated
1 cup sugar
½ cup all-purpose flour
pinch of salt
whipped cream for serving
Place rack in top third of oven and heat to 400 degrees. Butter a 9-inch springform pan and set aside. In a double boiler or microwave oven, melt together the butter and the chocolate. Stir to blend. (Using a Corningware pot, I microwaved for one minute.)
In a medium bowl, stir together egg yolks and sugar. Stir in flour. Add chocolate mixture and stir until smooth. Using an electric mixer, whisk egg whites and salt until stiff byt not dry. Fold whites into chocolate mixture just until blended. Pour into cake pan.
Bake for 25 minutes. Remove cake from oven and allow to cool for 1 hour. Wrap with foil and refrigerate until cake is firm and cold, at least 2 hours. Two hours before serving, remove cake from refrigerator and bring to room temperature. Slice (center of cake will be fudgy) and serve, if desired, with whipped cream.