Preheat your oven to 375°F (190°C). Line a 10×15 inch jelly roll pan with parchment paper, allowing it to hang slightly over the edges. Lightly grease the parchment.

Prepare a clean kitchen towel dusted with powdered sugar or cocoa powder for rolling the sponge later.

Separate egg whites and yolks into two mixing bowls. Beat yolks with granulated sugar for 3-4 minutes until thick and pale.

Add vanilla, melted butter, and milk to yolks, mixing briefly. Sift flour, cocoa, baking powder, and salt into the bowl and fold gently.

Whip egg whites until stiff peaks form, then gently fold into the chocolate mixture in two additions.

Pour batter into the prepared pan and spread evenly. Bake for 12-14 minutes until springy to the touch.

Immediately after baking, flip the hot sponge onto the prepared towel and peel off the parchment. Roll tightly with the towel and let it cool seam-side down.

Once cooled, whip the cream together with powdered sugar and vanilla until firm. Unroll the sponge gently, spread the filling, and re-roll without the towel.

Wrap the roll in plastic wrap and chill for at least 1 hour. Dust with powdered sugar before serving.

Slice using a serrated knife and serve immediately or refrigerate until ready to eat.