STEP 1: WARM THE SABAYON (DOUBLE BOILER)
Combine the whole eggs, sugar, and salt in a heatproof bowl set over a saucepan of gently simmering water (ensure the bottom of the bowl does not touch the water). Whisk constantly until the sugar has fully dissolved and the mixture reaches 110°F–115°F (43°C–46°C), warm to the touch.

STEP 2: WHIP TO THE RIBBON STAGE
Transfer the warm egg mixture to the stand mixer fitted with the whisk attachment. Whip on medium-high speed for 7–9 minutes until pale, quadrupled in volume, cool to the touch, and falling from the whisk in thick, slowly dissolving ribbons (le ruban). Whisk in the vanilla extract during the last 30 seconds.

STEP 3: FOLD IN THE DRY INGREDIENTS
Sift one-third of the flour-cornstarch mixture over the whipped eggs. Using a wide silicone spatula, gently fold from the center down to the bottom and up around the sides, rotating the bowl. Repeat with the remaining flour in two more additions, folding with minimal strokes until no dry pockets remain.

STEP 4: TEMPER THE BUTTER
In a small prep bowl, spoon approximately 1 cup of the aerated batter into the melted butter and whisk vigorously until emulsified. Pour this tempered mixture back around the edges of the main batter. Fold gently 3 to 4 times until just integrated, taking care not to deflate the delicate foam.

STEP 5: BAKE
Immediately pour the batter into the prepared pan, smoothing the surface with a single light pass of an offset spatula. Bake at 350°F (175°C) for 25–28 minutes, until golden, dry to a light touch, and springy in the center.

STEP 6: COOL
Let rest in the pan on a wire cooling rack for 10 minutes. Run a thin palette knife around the collar, invert onto the rack, peel away the parchment paper, and invert right-side up to cool completely.

ASSEMBLE
Once completely cooled, level the dome with a long serrated cake knife if necessary. Slice horizontally into two or three even layers using a cake leveler. Because classical génoise contains minimal fat, always brush each layer liberally with a 1:2 water to sugar ratio syrup flavored with citrus zest, vanilla, or liqueur) before spreading fillings, mousses, or buttercream.

TEMPERATURE MATTERS
Warming the eggs relaxes the proteins and liquefies the yolk fats, allowing the mixture to trap far more air and achieve maximum volume without chemical leaveners.

THE TEMPERING STEP
Liquid butter is denser than whipped egg foam and will sink directly to the bottom if added straight, collapsing your air structure. Tempering a small scoop of batter with the butter first ensures effortless, light incorporation.

STORAGE
Wrap un-syruped cooled sponge cakes tightly in plastic wrap. Keep at room temperature for up to 2 days, or freeze wrapped in plastic and aluminum foil for up to 2 months.