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1. Preheat the oven to 150°C. Butter and flour a square or rectangular cake tin (about 20 x 20cm) or line it with parchment paper.
2. Separate the egg whites from the yolks. In a large bowl, beat the egg yolks with the sugar until the mixture is white.
3. Add the melted butter, vanilla extract (if using), then stir in the speculoos powder and flour. Mix well.
4. Add the hot milk a little at a time while continuing to stir. You will have runny dough, this is normal.
5. Ride the snow whites with a pinch of salt until you have firm peaks.
6. Gently fold snow white into the dough three times, gently stirring with a spatula. It is normal for pieces of white to float to the surface, this is what will form the magical texture.
7. Pour the mixture into the tin and smooth the top with a spatula.
8. Bake for 50-60 minutes. The cake should be golden on top but still a little wobbly in the center.
9. Let cool completely at room temperature, add melted dark chocolate, then refrigerate for at least 4 hours (ideally overnight) for the layers to form.
10. Mold and cut into squares before serving. You can sprinkle a little icing sugar or decorate with crumbled speculoos.
Enjoy!