Cake

1. Preheat the oven to 180 °C / 350 °F. Grease a square pan and line it with parchment paper.
2. In a mixing bowl or stand mixer, beat the eggs with sugar until frothy and pale. If using a stand mixer, whip on high speed for about 5 minutes — this will give the cake a light, airy texture.
3. In a separate bowl, combine hazelnut butter (slightly melted if needed), milk, and oil. Mix until smooth and fully incorporated.
4. Add vinegar, baking powder, and baking soda to the wet mixture and stir gently.
5. Gradually add the flour to the wet mixture and mix until just combined.
6. Carefully fold the whipped eggs into the wet mixture using a spatula, folding gently to preserve the airiness of the batter.
7. Pour the batter into the prepared pan and smooth the top with a spatula.
8. Bake for 30–40 minutes at 180 °C / 350 °F, or until a toothpick inserted in the center comes out clean.
9. Let the cake cool completely on a cooling rack before adding the mousse.

Mousse

1. Hydrate the gelatin sheets in cold water until soft.
2. In a small saucepan, heat the milk with half of the sugar until warm.
3. In a separate bowl, place the egg yolks. While whisking constantly, slowly pour in the warm milk. This ensures the eggs don’t scramble.
4. Return the mixture to the stove over very low heat and stir constantly until it reaches 71–74 °C / 160–165 °F, which is the safe temperature for eggs. Do not let it boil. Remove from heat and let it cool slightly.
5. Add the gelatin sheets. Stir in the hazelnut paste until fully combined. 
6. In a separate clean, dry bowl, whip the cream with the remaining sugar until soft peaks form.
7. Gently fold the whipped cream into the hazelnut custard mixture in multiple additions until smooth and airy.
8. If necessary, trim the top of the cake to make it level, then pour the mousse evenly over the cake layer.
9. Smooth the top with an offset spatula and chill for 8 hours or overnight until fully set

Topping

1. Spread white chocolate-hazelnut spread evenly over the set mousse layer.
2. Sprinkle crushed hazelnuts on top for texture and flavor (optional).
3. Chill briefly before serving to allow the topping to set.

Serving & Presentation

1. Slice carefully using a hot, clean knife for neat pieces.
2. Serve chilled, optionally with whipped cream, caramel drizzle, or fresh seasonal fruits.