3 tablespoons (42 grams) hot melted unsalted butter
1/2 cup (65 grams) cake flour
1/4 teaspoon salt
1/3 cup (30 grams) unsweetened regular or Dutch-processed cocoa powder
4 large eggs
2/3 cup (135 grams) granulated white sugar
1 teaspoon pure vanilla extract
Preheat oven to 350 degrees F (180 degrees C). Butter, or spray with a non stick spray, a 9 inch (23 cm) round cake pan and line the bottom of the pan with parchment paper.
Sift the flour with the salt and cocoa powder.
In a heatproof bowl whisk together the eggs and sugar. Place the bowl over a saucepan of simmering water. Whisking constantly, heat the eggs and sugar until lukewarm to the touch (this will take about 5 minutes). Remove from heat and transfer the mixture to the bowl of your electric mixer. Beat on high speed (about 3-5 minutes) until the egg mixture has cooled, tripled in volume, and looks like softly whipped cream (the batter will fall back into the bowl in a ribbon-like pattern). Beat in the vanilla extract.
Then sift about one-third of the flour mixture over the whipped eggs and gently fold in using a large rubber spatula or whisk. Fold in half of the remaining flour, and then fold in the rest. (Do not over mix or the batter will deflate). Take about 1 cup of the batter and fold it into the hot butter mixture.
Then, with a spatula, gently fold the butter mixture completely into the egg batter. Pour into your prepared pan, smoothing the top. Bake until the cake shrinks slightly from the edges of the pan and the top springs back when lightly pressed (about 20 - 25 minutes). (A toothpick inserted into the center of the cake comes out clean.) Cool on a metal rack.
When the cake has cooled completely, run a small knife around the edges to release the cake. The genoise will keep two days in the refrigerator or it can be frozen for a couple of months.