Mousse Cake Recipe

Make caramel

In a medium heavy-bottomed saucepan, combine 1 cup granulated sugar and 2 tablespoons water. Stir to moisten the sugar, then set over medium heat without stirring. Swirl the pan occasionally until the sugar melts and turns an amber color (about 6–8 minutes). Watch carefully to avoid burning.

Once amber, remove from heat and carefully whisk in 3 tablespoons heavy cream (mixture will bubble vigorously). Add 2 tablespoons unsalted butter and stir until smooth. Stir in 1 teaspoon fleur de sel. Transfer the caramel to a small bowl and let cool to warm room temperature while you continue (it will thicken). Reserve about 3–4 tablespoons for a light caramel layer and the rest can be used for garnish or additional layering.

Prepare pan and batter

Preheat oven to 350°F (175°C). Line the bottom of an 8-inch (20 cm) removable-bottom or springform ring with parchment, and oil the ring sides or line with acetate if available.

In a small bowl whisk together 3 tablespoons flour, 1 tablespoon cocoa powder and a pinch of salt. In a separate bowl, beat 2 large eggs with 3 tablespoons sugar and 1 teaspoon vanilla extract until pale, thick and slightly ribboning (about 3–4 minutes by hand or 2–3 minutes with a mixer). Fold the dry ingredients into the eggs gently, being careful not to deflate the batter.

Spread the batter evenly in the prepared ring to make a thin layer (it should be about 1/4-inch thick). Bake 8–10 minutes until set and springy. Remove from oven and cool completely on a rack. Keep the ring in place; this will form the cake base.

Bloom gelatin

Sprinkle 1 teaspoon gelatin powder over 2 tablespoons cold water in a small bowl and let sit 5–7 minutes to bloom.

Temper chocolate with milk and yolks

Place 9 oz chopped bittersweet chocolate in a heatproof bowl. In a small saucepan, heat 1 cup whole milk until it starts steaming but not boiling. In a bowl whisk 4 egg yolks with 1.25 tablespoons sugar until smooth. Slowly pour about 1/3 of the hot milk into the egg yolks, whisking constantly to temper. Return this mixture to the saucepan with the remaining milk and heat over low, stirring, until the mixture is warm and slightly thickened (do not boil) — it should coat the back of a spoon.

Pour the warm milk and yolk mixture over the chopped chocolate and let sit 1 minute, then whisk until smooth and glossy. Stir in the bloomed gelatin until fully dissolved. Allow this chocolate base to cool to about room temperature (warm but not hot) — about 10–15 minutes.

Whip cream and combine

While the chocolate base cools, whip 1.5 cups cold heavy cream to soft peaks. Fold one-third of the whipped cream into the chocolate base to loosen it, then gently fold in the remaining whipped cream in two additions until homogeneous and airy. Do not overfold.

Pour about two-thirds of the mousse over the cooled sponge base in the ring, smoothing the top. Spoon the reserved cooled fleur de sel caramel (about 3–4 tablespoons) in a thin layer over the mousse, leaving a 1/2-inch border to avoid seepage, then pour the remaining mousse over to fully enclose the caramel. Smooth the top. Tap gently to remove air pockets.

Cover and refrigerate the assembled cake for at least 4 hours, preferably overnight, until set.

Make glaze

About 30 minutes before serving, heat 1/2 cup heavy cream in a small saucepan until it just begins to simmer. Place 4 oz chopped bittersweet chocolate in a bowl and pour the hot cream over it. Let sit 1 minute, then whisk until smooth. Stir in 1 tablespoon unsalted butter until glossy. Let the glaze cool slightly until just pourable but not hot (about 5–10 minutes).

Remove the cake from the ring and parchment or acetate and place on a rack over a sheet pan. Pour the glaze over the chilled mousse cake, starting in the center and allowing it to flow to the edges to create a smooth finish. Use a spatula to nudge any drips if needed. Return to the refrigerator for 10–15 minutes to set the glaze.

Once glaze is set, transfer the cake to a serving plate. Sprinkle 1/2 teaspoon extra fleur de sel lightly over the top and garnish with chocolate shavings if desired (about 2 tablespoons).

Slice with a hot, dry knife (wipe between cuts) and serve chilled. Store any leftovers refrigerated for up to 3 days.