8-inch (20 cm) round cake pan (at least 2 inches tall)
Stand mixer or hand mixer
Fine-mesh sieve, rubber spatula, parchment paper
Preheat the oven to 350°F (175°C).
Line the bottom of an 8-inch round pan with parchment. Do not grease the sides; the batter needs traction to climb and rise.
In a small bowl, whisk cocoa with hot water until smooth and glossy. Stir in the vanilla and oil. Set aside to cool slightly.
Add eggs, sugar, and salt to a large bowl. Beat on medium-high for 6–8 minutes until the mixture triples in volume, looks pale, and forms thick ribbons that sit on the surface for a few seconds before melting back in.
If the bowl feels cool or the eggs looked chilled, go the full time to build structure.
In a separate bowl, sift together flour, cornstarch, and baking powder. Sift twice for extra lightness.
Stream the cocoa mixture around the edges of the whipped eggs. Fold gently with a spatula 6–8 turns.
Sift in the dry ingredients in 2 additions, folding with broad, gentle strokes from the bottom up. Rotate the bowl as you fold.
Stop folding when no dry streaks remain. Overmixing deflates the batter and leads to a flat cake.
Pour the batter into the prepared pan. Tap once on the counter to release large bubbles, but don’t overdo it.
Bake for 22–28 minutes, until the top springs back and a skewer comes out with a few moist crumbs. Avoid opening the oven for the first 18 minutes.
Remove from the oven and run a thin knife around the edge to loosen just the top rim if it clings. Cool in the pan for 5 minutes.
Invert onto a rack, peel off parchment, then turn upright to finish cooling. Let it reach room temperature before slicing or frosting.
Keep it simple: Dust with cocoa or powdered sugar.
Dress it up: Layer with lightly sweetened whipped cream and berries.
Go classic: Brush layers with coffee syrup, then fill with chocolate whipped cream.
