Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper and set aside.
In a large mixing bowl, beat the eggs and sugar together with an electric mixer on high speed until the mixture becomes thick, pale, and doubles in volume — about 5 minutes. This step helps create a fluffy sponge.
Sift together the cocoa powder, flour, baking powder, and salt. Gently fold the dry ingredients into the egg mixture, mixing just until combined to preserve the airiness of the batter.
Pour the batter into the prepared baking sheet, spreading it evenly with a spatula to ensure a smooth, level surface.
Bake for 10-12 minutes, or until the cake is firm to the touch and a toothpick inserted in the center comes out clean. The sponge should be lightly fragrant with cocoa.
Once baked, remove the cake from the oven and invert it onto a clean kitchen towel dusted with cocoa powder. Carefully peel off the parchment paper to reveal the cooked sponge.
While the sponge is still warm, gently roll it up from one short end, using the towel to help shape it into a tight log. Let it cool completely, about 20 minutes, to set its shape.
Meanwhile, prepare the ganache by heating the heavy cream until just bubbling — don’t boil. Pour over the chopped chocolate and let sit for 2 minutes.
Stir the mixture until smooth and glossy, then add the butter and stir until fully incorporated. Allow the ganache to cool slightly until it thickens but remains spreadable.
Unroll the cooled sponge carefully, then spread a generous layer of ganache evenly over the surface.
Gently re-roll the sponge into a tight spiral, smoothing out the outer surface as needed. Place on a serving platter.
Slice the roll into individual portions with a sharp knife and enjoy. The cake should be moist, with a soft interior and a shiny chocolate exterior.
