Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Separate the eggs, placing egg whites and yolks into separate bowls. Beat the egg whites with half of the sugar until stiff peaks form and become glossy.
In another bowl, whisk the egg yolks with the remaining sugar and vanilla until pale and fluffy.
Gently fold the whipped egg whites into the yolk mixture in several additions, keeping the batter airy.
Sift together flour, cocoa powder, baking powder, and salt, then gently fold into the batter until smooth but still light.
Pour the batter onto the prepared baking sheet, spreading it evenly to about 1/4 inch thick with a spatula.
Bake for 12-15 minutes, or until the cake feels springy to the touch and a toothpick inserted comes out clean.
Once baked, immediately turn the cake out onto a clean kitchen towel dusted with cocoa powder. Carefully peel off the parchment paper.
Starting from a short end, gently roll the cake with the towel to form a loose spiral. Let it cool completely in this shape to prevent cracking.
Whip the heavy cream with powdered sugar and a pinch of salt until soft peaks form, making a fluffy filling.
Unroll the cooled cake carefully, spread the whipped cream evenly over the surface, and sprinkle with a little cocoa powder for contrast.
Gently re-roll the cake into its spiral shape, smoothing slightly as needed. Slice and serve to reveal the beautiful layers inside.
