Preheat oven to 350°F (180°C)

Sift together flour, cocoa powder, cornstarch, and salt

Crack eggs into a large bowl and add sugar

Use a double boiler method: place bowl with eggs and sugar over hot water and whip on low speed, gradually increasing to high speed for about 10 minutes until light, fluffy, and forming soft peaks

Fold in dry ingredients gradually using a spatula, stirring from bottom to top to keep batter light and airy

Pour in boiling water and mix gently

Add vegetable oil and stir gently to combine

Pour batter into center of prepared 16x15 inch baking tray, spreading evenly to corners and edges

Bake for 12 minutes (adjust time based on batter thickness) until a toothpick inserted in center comes out clean

Immediately roll the hot sponge cake using parchment paper and a kitchen towel for support

Let the rolled cake cool completely

For ganache: chop dark chocolate finely and place in a bowl

Heat heavy cream over medium heat until steaming with small bubbles around edges

Pour hot cream over chopped chocolate and let sit briefly to melt

Stir gently with spatula until smooth and well combined

Spread ganache over cooled sponge cake, reserving about a quarter for topping

Roll up the sponge with ganache filling

Pour remaining ganache over the rolled cake and spread with spatula or silicone brush

Serve immediately or chill for a few hours before serving