Heat the oven to 180°C. Grease and line an 28cm x 40cm cake tin that is at least 2.5cm deep.
In a large bowl, whisk the egg yolks with an electric mixer until pale and slightly thick. Add sugar and whisk until stiff. Stir in vanilla then sift in cocoa and fold through.
In a another large bowl, whisk egg whites until they form stiff, glossy peaks. Mix whites into the yolks one spoonful at a time until fully combined.
Pour mixture into the prepared tin, spreading it evenly. Bake for 20 minutes or until it is set and slightly pulling away from the sides of the tin.