In the bowl of stand mixer, combine the four eggs with the sugar and beat with the whisk attachment for 5-6 minutes, or until the mixture has become thick and pale. When you pull out the whisk, any egg that runs off should leave a ribbon across the top of the mixture that stays visible for about 3-5 seconds.

While the eggs are beating, sift together the flour, cocoa powder, baking powder, and salt in a separate bowl. Melt the butter in a microwave safe bowl and set it aside to cool. After the eggs have finished beating, use a rubber spatula to gently fold in the dry ingredients. Adding the flour mixture a little bit at a time helps to incorporate it without lumps or pockets of flour staying unmixed. At the end, fold in the melted butter (make sure it has cooled, but not hardened).

Pour the mixture into the prepared baking sheet and spread it out into an even layer. Bake for 15-20 minutes. It will be ready when the edges are beginning to look dry, and a toothpick inserted in the center comes out clean or with a few moist crumbs. The center may not look completely cooked, but it is ready. Remove it from the oven, run a sharp knife around the edges, and allow it to sit for 5 minutes.

Using a sieve, sprinkle the top of the cake with cocoa powder. Then, lay another sheet of parchment over the top and flip the cake out onto it. Beginning on a short end, roll the cake tightly but gently— you don’t want to smash it! Allow it to cool completely before filling.