6 large eggs, separated
1 cup (226 grams) unsalted butter, cut into small pieces
1 cup (200 grams) granulated white sugar, divided
1 teaspoon pure vanilla extract
Preheat oven to 350 degrees F (177 degrees C) and place oven rack in the center of the oven. Butter a 9 x 3 inch (23 x 8 cm) pan or spray with a nonstick cooking spray.
Separate the eggs while still cold, placing the egg whites in one bowl and the egg yolks in another bowl. Cover both with plastic wrap and bring to room temperature (about 30 minutes).
Meanwhile, melt the butter and chocolate in a bowl placed over a saucepan of simmering water.
Place egg yolks and 1/2 cup sugar in the bowl of your electric mixer (can also use a hand mixer). Beat on medium high speed until thick and lemon-colored, about 3-5 minutes. (The eggs should have tripled in volume, look thick and soft, and when you lift the beater the mixture falls back into the bowl in a slow ribbon.) Beat in the vanilla extract and melted chocolate mixture.
In a clean bowl, with the whisk attachment, beat the egg whites until foamy. Gradually add the remaining 1/2 cup sugar and beat until stiff peaks form. Using a rubber spatula or whisk, fold a small amount of whites into the egg yolk mixture to lighten the batter. Add the remaining egg whites, folding just until incorporated. Do not over mix or the batter will deflate.
Pour into the prepared pan, smoothing the top. Bake the cake for about 50 - 60 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. (During baking the surface of the cake will form a crust which will collapse when the cake is removed from the oven.) Remove from oven and place on a wire rack to cool. The top of the cake will have become hard with a cracked surface and lots of crumbs. Serve with softly whipped cream. Enjoy! Buon Appetito!