Have all ingredients at room temperature, 68┬░ to 70┬░F. Preheat the oven to 350┬░F. Grease and flour the bottom(s) of two 9 x 2-inch round pans or one 9-inch springform pan or line the bottom(s) with wax or parchment paper.
Sift together 3 times and return to the sifter:
1/2 cup plus 1 tablespoon sifted cake flour
1/2 cup plus 1 tablespoon unsweetened cocoa
Melt in a small saucepan:
1/3 cup clarified butter
Whisk together in a large heatproof bowl:
6 large eggs
1 cup sugar
Set the bowl in a skillet of barely simmering water and whisk constantly until the mixture is warm to the touch (about 110┬░F). Remove the bowl from the heat and beat on high speed until the mixture is light colored, has tripled in volume, and is the consistency of softly whipped cream (3 to 5 minutes in a heavy-duty mixer with the whisk attachment, about 15 minutes with a hand-held mixer). In 3 additions, sift the flour mixture over the top and fold in with a rubber spatula. Reheat the butter until it is hot and transfer to a medium bowl. Fold into the batter with a rubber spatula until completely incorporated about 1 1/2 cups of the egg mixture along with:
1 teaspoon vanilla
Scrape this into the remaining egg mixture and fold in. Scrape the batter into the pan(s) and spread evenly. Bake until the cake begins to pull away from the sides of the pan(s) and the top springs back when lightly pressed, about 15 minutes in round pans, 30 to 35 minutes in a springform pan. Let cool in the pan(s) on a rack for 10 minutes. Slide a thin knife around the cake to detach it from the pan(s); remove the side of the springform pan, if using. Let cool right side up on the rack.
