Start by making the custard and melting the chocolate so they can cool slightly before assembling the cake.
First, in a saucepan, combine the milk and heavy cream with the lemon peel and cinnamon stick. Heat until just before boiling, then turn off the heat, cover, and let it infuse.

While the milk is infusing with the cinnamon and lemon, place the heavy cream and chopped chocolate in another saucepan. Stir over low heat until the chocolate is completely melted. Be careful not to let it stick or burn. Once melted, set aside to cool.

Separate the egg whites from the yolks. In a bowl with the yolks, add the sugar, vanilla sugar (or vanilla extract), and cornstarch. Mix well until the mixture is pale and smooth.

Gradually add the reserved milk mixture (after removing the cinnamon stick and lemon peel) to the yolk mixture, stirring constantly to prevent the eggs from curdling.

Once combined, pour the mixture back into the saucepan. Cook over medium-low heat, stirring constantly, until the custard thickens to your desired consistency. Set aside to cool while you finish the mousse.

Beat the egg whites until stiff peaks form. Gently fold them into the melted chocolate using slow, sweeping motions to keep the mixture light and airy.

Let the cake rest in the refrigerator for at least three hours. Then it’s ready to serve. Enjoy!