Prepare cake layer: Preheat oven to 350°F (180°C). Line a square baking pan with parchment paper.

Beat eggs & sugar: In a large bowl, whisk eggs and sugar on high speed for 5 minutes until pale and frothy.

Combine wet ingredients: In another bowl, mix hazelnut butter, milk, and oil until smooth. Add vinegar, baking powder, and soda.

Combine batter: Gently fold flour into the wet mixture, then fold in whipped egg mixture carefully to maintain airiness.

Bake: Pour batter into pan and smooth top. Bake 30–40 minutes until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.

Prepare mousse: Soak gelatin sheets in cold water to soften. In a saucepan, warm milk with half the sugar until dissolved. In a separate bowl, whisk yolks and gradually pour in warm milk. Return to low heat, stirring constantly, until mixture reaches 160–165°F (71–74°C). Remove from heat.

Add gelatin & hazelnut butter: Stir softened gelatin into warm custard until dissolved, then mix in hazelnut butter until smooth. Let cool slightly.

Fold whipped cream: Whip heavy cream with remaining sugar to soft peaks, then gently fold into cooled hazelnut custard until light and uniform.

Assemble cake: Trim cake top if uneven. Spread mousse evenly over cake layer and smooth top with an offset spatula. Refrigerate at least 8 hours or overnight until set.

Add topping: Spread white chocolate‑hazelnut spread evenly over set mousse and sprinkle crushed hazelnuts on top. Chill briefly to firm before slicing.

Serve: Warm a knife under hot water, wipe dry, and slice for clean portions.