1. Start with the custard by mixing the cornstarch, sugar, and vanilla extract with some milk in a small bowl until well combined. Pour the rest of the milk with the eggnog into a small saucepan and heat up on the stove, stirring constantly. As soon as the milk starts to bubble, stir in the starch mixture. Allow the custard to thicken, stirring constantly. The custard will be slightly thicker/firmer than usual. Remove from the heat and stir in the two egg yolks. Place a piece of plastic wrap directly on top of the custard to prevent skin from forming, and let cool down.

2. Mix the flour with sugar, cocoa powder, and baking powder for the chocolate streusels in a large bowl. Add the soft butter (or margarine), eggs, and vanilla extract and mix with a fork/spoon fist, then with your fingers to create different-sized streusels.

3. For the regular streusels, mix flour with cane sugar, then add the soft butter (or margarine) and vanilla extract. Mix to get different-sized streusels.

4. Preheat the oven to 350°F (180°C). Line a 9.5-inch (24cm) springform tin* with baking paper and grease lightly. Add about 2/3 of the dark streusels to the baking tin and press to the bottom and sides. If you like, you can sprinkle a few more of the streusels loosely over the base. Stir the custard again to get a smoother texture, then spoon evenly over the crumble base. Sprinkle the remaining dark and light sprinkles over the custard – either in the pattern I did or whatever way you like.

5. Bake the cake in the center of the oven for about 40-45 minutes. Remove the cake from the oven and let cool down completely inside the tin. When cooled, run a knife around the cake and remove it carefully from the baking tin. Pull the cake with the baking parchment onto a serving plate (and then pull the paper out from under the cake, of course), and serve.