Whisk Dry Ingredients: In a mixing bowl, combine the cake flour, cocoa powder, vanilla sugar, and ¾ cup of the granulated sugar. Whisk well and set aside.

Whip the Egg Whites: In a large bowl, beat the 14 egg whites with an electric mixer for about 1 minute until foamy. Gradually add the remaining ¾ cup of granulated sugar while continuing to beat on high speed until the whites hold soft peaks.

Fold the Batter: Add the dry flour mixture to the beaten egg whites, one-third at a time. Gently fold after each addition using a rubber spatula just until the batter is fully incorporated.

Bake: Spoon the batter evenly into the prepared ring cake tin. Bake for 40 to 45 minutes, or until a skewer inserted into the center comes out clean. (Tip: Start checking the cake at 35 minutes, as ovens can vary). Remove from the oven and let it cool completely on a wire rack.

Invert the Cake: Carefully invert the cooled cake onto a flat serving dish and lift the tin straight up.

Prepare the Mascarpone Filling: In a medium bowl, beat the heavy cream and 2 tablespoons of sugar with an electric mixer until soft peaks form. Gently fold in the mascarpone cheese using a spatula until the mixture is uniform and homogenized. Do not over-whip; treat it gently to maintain a smooth texture. Refrigerate until ready to assemble.

Create the Cake Tunnel: Slice a 2.5 cm (1-inch) layer horizontally off the top of the cake and set it aside. To create a tunnel, carefully hollow out the bottom section of the cake, leaving a 2.5 cm (1-inch) shell on the sides and bottom. You can use a melon baller or spoon to scoop out the sponge cake crumbs.

Assemble: Spoon the Nutella evenly into the hollowed-out tunnel, then fill the rest of the cavity with the chilled mascarpone cheese filling. Carefully replace the sliced top layer back onto the cake.

Make the Chocolate Ganache: In a small saucepan, combine the semi-sweet chocolate and heavy cream over medium-low heat. Whisk continuously until the chocolate is fully melted, smooth, and glossy.

Glaze & Serve: Spoon the warm ganache over the entire surface of the assembled cake, letting it drip beautifully down the sides. Allow the glaze to set slightly before slicing. Enjoy!