Preheat the oven to 180° C/ 356° F.
Start preparing the cake sponge by cracking the eggs and separating the yolks from the whites.
Whisk the egg whites with a hand mixer until they turn white and form stiff peaks. Place them in the fridge while we work with the other ingredients.
In a separate bowl, mix the egg yolks together with the brown sugar on medium speed until creamy and glossy. Pour in the milk, margarine and chocolate powder on top and mix thoroughly for about 3-4 minutes.
Combine the flour together with the baking powder and gradually incorporate in the egg yolk mixture at a low mixing speed to avoid spilling it.
Continue mixing until you get a smooth batter, free of clumps.
Take the egg whites out from the fridge and transfer to the batter. Carefully fold them into the batter with a spatula in upward movements to avoid deflating the mixture.
Prepare a large baking tray of about 36 x 30 cm (12 x 14 inches). Grease the bottom and sides and cover with parchment paper.
Pour the sponge batter in the tray and level it out using a spatula or a spoon.
Place in the oven on the middle rack and bake for approximately 20 minutes, or until golden. Check with a toothpick to see when the sponge is ready. If you insert the toothpick in the center of the sponge and it comes out clean, it means it's ready to take out of the oven.
When the cake is done, take it out and let it cool slightly. Remove from the tray by placing a clean towel over the cake and holding gently while you upturn the tray with the other hand. Remove the parchment paper and carefully roll it up in the kitchen towel and set aside.
While the cake sponge cools on the side, let's prepare the filling.