Preheat the oven to 230°C. Grease a 17×12-inch baking pan, line with parchment paper, then butter and flour the paper.

Sift flour, cocoa, and cornflour into a bowl and set aside.

Separate two eggs, placing the whites into a clean, dry bowl.

In another bowl, beat the two egg yolks, three whole eggs, and ½ cup sugar on high speed until pale and fluffy, then mix in the vanilla.

Gently fold the sifted dry ingredients into the egg mixture in two batches, mixing only until combined.

Beat the egg whites until foamy, add 2 tablespoons sugar, and continue beating until stiff peaks form.

Fold the egg whites into the batter in three batches, keeping the mixture light and airy. Spread the batter evenly into the prepared pan.

Bake for 6–8 minutes or until lightly golden. Remove from the oven, dust the top with powdered sugar, invert onto a towel, peel off the parchment, dust again with sugar, and roll the cake tightly in the towel. Cool on a wire rack.

For the filling, mix melted chocolate with heavy cream until smooth and slightly thickened.

Unroll the cooled sponge, spread the chocolate filling evenly, and roll it up again. Chill for a few hours to set.

Before serving, dust with powdered sugar and finish with extra ganache or decorative toppings like meringue mushrooms.