Hazelnut Mousse Cake Recipe

Prepare and preheat

Preheat the oven to 350°F (175°C). Line the bottom of a 9-inch (23 cm) springform pan with parchment and lightly butter the sides or spray with baking spray.

Whisk together the all-purpose flour, ground toasted hazelnuts, baking powder and salt in a bowl until evenly combined.

In a separate large bowl, whisk the eggs and granulated sugar until pale and slightly thickened (about 3–4 minutes by hand or 2 minutes using a mixer). Add the vanilla extract and melted cooled butter and fold gently to combine.

Fold the dry ingredients into the egg mixture in two additions, using a spatula to keep as much air in the batter as possible without overmixing. The batter will be relatively loose but aerated.

Pour the batter into the prepared springform pan and smooth the top. Bake for 18–22 minutes or until the top is golden and a toothpick inserted near the center comes out with a few moist crumbs. Remove from oven and cool in the pan on a rack for 10 minutes, then release the springform and transfer the sponge to a rack to cool completely. Once cooled, if needed, trim the dome to create an even layer that will fit the mousse ring or pan.

Bloom the gelatin: sprinkle the powdered gelatin over the cold water in a small bowl and let sit for 5–8 minutes until spongy.

Warm the milk and half of the sugar in a small saucepan over medium heat until it just begins to steam (do not boil).

While the milk warms, whisk the egg yolks with the remaining sugar in a metal or heatproof bowl until smooth.

Temper the yolks: slowly pour about 1/3 of the hot milk into the egg yolks while whisking continuously to raise their temperature. Then pour the tempered yolk mixture back into the saucepan with the remaining milk.

Cook the mixture over low heat, stirring constantly with a wooden spoon or silicone spatula, until the custard thickens slightly and coats the back of the spoon (about 3–5 minutes). Do not let it boil.

Remove the custard from heat and stir in the bloomed gelatin, whisking until fully dissolved. Add the hazelnut praline paste (or hazelnut-syrup mixture) and mix until smooth. Stir in the vanilla and salt. Let the mixture cool to room temperature, stirring occasionally to prevent a skin from forming.

Meanwhile, whip the cold heavy cream in a chilled bowl to soft peaks. The cream should be smooth and billowy but still foldable.

When the hazelnut custard is cooled but still pourable (around room temperature, not set), fold about one-third of the whipped cream into the custard to lighten it. Then gently fold in the remaining cream in two additions until the mousse is homogeneous and airy. Do not overfold.

Place the cooled hazelnut sponge back into the 9-inch springform pan or a mousse ring set on a parchment-lined tray. Pour the mousse over the sponge, smoothing the top with an offset spatula. Tap lightly to remove large air pockets. Chill in the refrigerator for at least 4 hours, ideally overnight, until the mousse is set.

Make the glaze: place the chopped dark chocolate in a heatproof bowl. Heat the 1/2 cup heavy cream in a small saucepan until it just reaches a simmer. Pour the hot cream over the chocolate and let sit for 1 minute, then stir gently from the center until fully smooth and glossy. Stir in the butter and sea salt until incorporated. If the glaze is too thick, warm slightly; if too thin, let it cool a few minutes.

Remove the chilled mousse cake from the pan or ring carefully. If you used a springform, release it; if using a ring, run a hot towel around the ring to loosen then lift off gently. Place the cake on a wire rack set over a tray to catch excess glaze.

Pour the chocolate glaze over the top, starting at the center and working outward to create an even coat. Use an offset spatula or spoon to nudge glaze over the edges for a smooth finish. Allow excess to drip off.

Transfer the cake to a serving plate and decorate with toasted hazelnut halves around the edge or in the center. Optionally sprinkle a small pinch of flaky sea salt over the glaze. Chill the cake 15–30 minutes for the glaze to set before serving.