Preheat your oven to 180°C (350°F). Line a baking tray with parchment paper and set aside.
Separate the eggs, placing the yolks and whites into separate bowls. Whisk the egg yolks with half of the sugar until pale and slightly thickened.
In a clean bowl, beat the egg whites with the remaining sugar until stiff peaks form and they become glossy.
Gently fold the cocoa powder, flour, baking powder, and salt into the egg yolk mixture, ensuring no lumps remain.
Carefully fold in a third of the whipped egg whites to loosen the batter, then gently fold in the remaining whites until smooth and airy.
Pour the batter into the prepared tray and spread evenly with a spatula. Bake for 12-15 minutes, or until the sponge springs back when gently touched.
Once baked, invert the sponge onto a clean kitchen towel dusted with cocoa powder. Carefully peel off the parchment paper and let it cool for a few minutes.
Meanwhile, heat the cream until it just starts to simmer, then pour over the chopped dark chocolate. Let sit for a minute, then stir until smooth to create a ganache.
Spread the chocolate ganache evenly over the slightly cooled sponge, leaving a small border around the edges.
Using the towel to gently roll the sponge from one short edge to the other, creating a tight spiral. Let it roll up completely and cool to set its shape.
Once cooled, unroll the cake slightly and spread a thin layer of whipped cream or frosting inside, then re-roll the cake gently.
Dust the top with cocoa powder or powdered sugar before slicing. Serve slices, revealing the beautiful spiral of cake and filling inside.
