Beat on low speed for 1 minute. Then, increase the mixer speed to medium (not high) and beat until the mixture increases in volume 4 1/2 to 5 times and resembles a bowl of slightly translucent soft foam composed of tiny bubbles. (This takes anywhere from 1 1/2 to 3 minutes.) The foam will hold a very soft, moist shape when the beaters are lifted. Beat in gradually (on medium speed), 1 tablespoon at a time, taking 2 to 3 minutes:
3/4 cup sugar
When all the sugar has been added, the foam will be creamy white and hold soft, moist, glossy peaks that bend over at the points; do not beat until stiff. If the mixer bowl is nearly full, transfer the mixture to a wide 4- to 6-quart bowl for easier folding. Sift a fine layer of the flour mixture (about one-eighth of the total) evenly over the surface of the batter and fold gently with a rubber spatula only until the flour is almost incorporated. Do not stir or mix. Repeat 7 more times, folding in with the last addition:
6 tablespoons finely chopped semisweet chocolate or bittersweet chocolate (optional)
Fold in until no traces of flour are visible. Pour the batter into the pan and tilt or spread to level the top. Bake until a toothpick inserted in the center comes out clean, 35 to 40 minutes. Let cool upside down for at least 1 1/2 hours, setting the tube over a bottleneck or resting the pan on 4 glasses. Unmold.
