Grease a 9x15 inch sheet pan (0.5 inch in height), and line the bottom and the sides with parchment paper. Preheat the oven to 190 degrees Celsius.
Sift the flour and cocoa powder together thrice. Set aside.
Place the eggs in a metal pot. Whisk in the sugar until combined. Set the pot over gentle heat and whisk constantly, until the mixture is foamy and the temperature is warm, that of baby's milk. Remove from heat and transfer the mixture into a mixing bowl.
Whip the mixture with an electric mixer on high speed until the mixture is no longer foaming and the bubbles disappeared. Whip until the mixture has tripled in volume and is thick and rich. Switch to the lowest speed and whip for 1 minute, until the mixture leaves a trail for a short while, before merging with the mixture.
Sift the flour mixture over the egg mixture in three parts, gently folding the flour into the egg mixture until thoroughly combined. There shouldn't be any flour pockets visible and do not over-mix.
Pour a couple tablespoonfuls of cake batter into the hot, melted butter and fold until the butter has been incorporated into the cake batter. Pour the mixture back into the cake batter and fold it in gently.
Transfer the cake batter into the sheet pan. Tilt the sheet pan so the batter spreads out evenly and drop it several times on the counter top to burst the uneven air bubbles.
Bake in the middle rack for 10 to 15 minutes until the cake springs back when touched gently and a skewer inserted into the centre of the cake comes out dry.
Whipped Cream and Assembly
While the cake is baking, prepare a parchment paper and dust it generously with icing sugar. Turn out the cake onto the parchment paper and peel away the parchment paper used to line the sheet pan. Roll the cake tightly (be careful, it's hot) and leave it on a rack to cool completely.
While the cake is cooling (or when the cake is cooled), beat the cream with icing sugar (till your desired sweetness) until stiff peaks form. You can flavour the whipped cream with vanilla, melted chocolate, coffee paste etc.
Unroll the cake and spread the cake with the whipped cream, and rolled it up again. Dust the top with icing sugar (optional) and serve in slices.