1. Preheat the oven to 200°C (390°F). Line a 24cm (9.5 inches) springform tin with baking parchment and set aside.

2. Mix the flour for the base with the sugar, cocoa, and salt in a large bowl. Add the cold butter and cut into pea-sized pieces with a pastry blender. Add the egg and water and mix with a fork, then knead briefly with your hands to get a smooth dough. Shape into a disk, place in the prepared tin, and press down to get a smooth layer. Bake for about 18-20 minutes. Remove from oven and let cool down in the tin. Do not turn off the oven.

3. While the cake base is in the oven, you can prepare the choux pastry. Add water, butter, and salt for choux pastry to a small saucepan and bring to a boil until the butter is completely melted. Mix the flour and starch together, then add to the pot in one go and stir until you have a smooth dough that pulls away from the pot. Continue stirring for about 1 minute – the bottom of the pot will be covered with a white layer that shows you the pastry dough is perfect. Transfer the pastry dough to a mixing bowl and let cool down for a few minutes. Add the eggs to the bowl (one at a time) and work them into the dough with dough hooks. The dough should be shiny and hang down in a triangle from the dough hooks when you pull them out. Fill into a piping bag fitted with a round nozzle. Line a baking sheet with baking parchment, then pipe small portions of the dough onto the prepared baking sheet, spacing them slightly apart – there should be enough dough for about 18 small cream puffs. Spray or brush with some water, smoothing out any tips peaking out if necessary. When the cake base is done, bake the cream puffs immediately afterward for about 20-23 minutes. Do not open the oven during this time or the pastry will deflate! Remove the finished cream puffs from the oven and let them cool down on a wire rack.

4. Poke small holes in the bottoms of the cooled cream puffs with a knife or wooden skewer. Whisk the heavy cream until stiff peaks form, transfer to a piping bag with a small round tip and fill the cream puffs with it. Set aside.

5. Coarsely chop the chocolate and melt it together with the butter, cocoa, and vanilla extract in a large (heatproof) bowl over a pot with simmering water. Remove the chocolate sauce from the stove and let cool down for about 10 minutes. Meanwhile, whisk the cold heavy cream until stiff peaks form. Add to the bowl with the cooled (lukewarm) chocolate in several batches and fold in. The first portion of whipped cream can be stirred in more vigorously, but after that, the whipped cream should be folded in very carefully to retain as much volume as possible.

6. Place the cake base on a serving plate and place a cake ring around it. Spread about 1/4 of the chocolate mousse on the base, then place the cream puffs on top with some space in between them – I could fit in 12 cream puffs – and then pour the remaining chocolate mousse over the cream puffs. Depending on how firm the mousse already is, you may need to press the mousse into the spaces between the cream puffs so there are no air bubbles. Smooth out the top and place in the fridge for about 2 hours.

7. Warm the cake ring slightly with a warm cloth, then slide a sharp knife between the ring and the cake and carefully loosen the ring. If the edges look a bit rough, you can smooth them out with a warm cake palette (the mousse melts slightly due to the heat and is easier to spread). If you like, you can now press some grated chocolate onto the sides of the cake (optional). Whisk the heavy cream for the decoration with the cream stiffener until it stiff peaks form. Fill into a piping bag with a star tip and decorate the cake. Place back in the fridge.

8. For the Eierlikör layer, soak the gelatin leaf in cold water for about 5 minutes, then add to a saucepan and heat up until dissolved. Gradually add the eggnog and mix well. Pour the mixture over the cake and spread evenly. Place in the fridge for another 2 hours.