Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper. Sift together cocoa powder, flour, baking powder, and salt in a bowl to ensure even distribution.
In a separate large bowl, whisk eggs and sugar until the mixture becomes pale, thick, and slightly fluffy, about 3-4 minutes. This aerates the eggs and helps create a light sponge.
Add vegetable oil and vanilla extract to the egg mixture, gently folding until combined. The batter should be smooth and shiny.
Gradually sift the dry ingredients into the wet mixture, folding carefully to maintain airiness and avoid deflating the batter.
Pour the batter onto the prepared baking sheet, spreading it evenly with a spatula until it forms a thin layer that covers the surface. Tap the pan lightly to settle the batter and remove air bubbles.
Bake in the preheated oven for about 12-15 minutes, or until the cake is set and a toothpick inserted in the center comes out clean. The surface should be glossy and slightly firm.
Remove the cake from the oven and immediately invert onto a clean kitchen towel dusted with powdered sugar. Carefully peel away the parchment paper.
While still warm, gently roll the cake along with the towel from one short end to form a tight spiral. Let it cool completely in this rolled shape — this helps it keep its flexibility.
Prepare the filling by whipping heavy cream, powdered sugar, and vanilla until soft peaks form, creating a fluffy, sweet cream.
Unroll the cooled cake carefully, then spread the creamy filling evenly over the surface, leaving a small border around the edges to prevent overflow when rolling again.
Gently re-roll the cake along the same spiral, pressing slightly to ensure the filling is enclosed. Trim the ends for a neat presentation if desired.
Slice the cake roll into portions, revealing the spiral pattern with a glossy chocolate exterior and creamy interior. Serve and enjoy this classic treat!
