Start by combining the eggs and vanilla in a large mixing bowl and beat on high speed for 2 to 3 minutes until well mixed. Add the sugar and continue beating for at least 5 minutes or until the mixture doubles in size and turns light and creamy in color.
In a separate bowl, sift together the flour, cocoa powder, and baking powder to ensure they are evenly mixed and free of lumps.
Fold the dry ingredients gently into the egg mixture to keep the batter airy. Use a spatula or mixer on a low setting for this step.
Line a baking tray with parchment paper and evenly spread the batter over it with a spatula. Bake at 180°C (350°F) for 10 to 12 minutes, until the cake is springy to the touch and slightly shrinks from the sides.
Immediately transfer the cake to a wet towel, rest for 5 minutes, then wrap with the towel and let it cool at room temperature. This makes the cake flexible for rolling later.
Unwrap the cake, spread the Swiss meringue chocolate buttercream evenly, then carefully roll the cake back up. Cut into desired sizes of logs, place them on your serving dish or cake board. Use a little buttercream to attach each log together.
Cover the logs with the remaining buttercream and decorate as desired. Check out my video tutorial for decorating tips.