Preparation Steps

• Line the base of an 8-inch springform pan with parchment and lightly butter the sides. Cut an acetate collar to fit the inside of the pan (tall enough to extend about 2 inches above the rim) and set it aside for assembly.
• Bloom the gelatin for the white mousse: sprinkle gelatin over the cold water in a small bowl and let stand 5 minutes. Place white chocolate, vanilla, and a pinch of salt in a medium heatproof bowl. Heat 1/2 cup cream until steaming; whisk the bloomed gelatin into the hot cream to dissolve, then pour over the white chocolate. Let sit 2 minutes, then whisk smooth. Press plastic wrap directly on the surface and let cool to room temperature (about 75–80°F/24–27°C).
• Heat oven to 340°F (170°C). In a bowl, whisk flour, sifted cocoa, baking soda, salt, and espresso powder for the cake base.
• In a separate bowl, whisk the melted butter, oil, and brown sugar until glossy. Whisk in sour cream, egg, and vanilla, then the milk until smooth.
• Add the dry mix to the wet and stir just until no dry streaks remain. Stream in the hot coffee (or water) and whisk until the batter is smooth and fluid.
• Scrape batter into the prepared pan and tap to release air bubbles. Bake 16–20 minutes, or until a toothpick comes out with a few moist crumbs. Cool 10 minutes, release the ring, invert onto a rack, peel parchment, and trim any dome. Return the cake to the cleaned springform base, lock on the ring, and insert the acetate collar snugly around the cake.
• Make the dark fudge layer: put chocolate chips in a bowl. Heat cream with honey and a pinch of salt just to a bare simmer; remove from heat, stir in vanilla, and pour over the chocolate. Let sit 1 minute, then whisk until glossy and smooth.
• Pour the warm fudge over the cake and tilt the pan to level. Set in the freezer for 12–15 minutes to firm the surface (it should no longer jiggle), then keep chilled while you prepare the mousse.
• For the dark chocolate mousse, gently melt the semisweet chocolate (microwave at 50% power in short bursts, stirring between each, or over a bain-marie). Keep it warm but not hot.
• Whisk yolks, sugar, and water in a heatproof bowl. Set over barely simmering water and whisk constantly until thickened, pale, and 165°F (74°C), 4–6 minutes. Remove from heat and whisk for 30 seconds to cool slightly, then slowly whisk this sabayon into the melted chocolate until completely smooth. Let cool to lukewarm (about 85°F/29°C).
• Whip 1 1/2 cups cold cream with powdered sugar and espresso powder to soft–medium peaks. Fold one-third into the chocolate base to lighten, then fold in the rest in two additions until no streaks remain.
• Spread the dark chocolate mousse over the set fudge in an even layer. Chill 30 minutes to start setting.
• Finish the white chocolate mousse: whip 1 1/4 cups cold cream with powdered sugar to medium peaks. Ensure the white chocolate–gelatin mixture is cool and thick like yogurt (not warm). Fold it into the whipped cream in two additions until silky and uniform.
• Spoon the white chocolate mousse over the dark mousse, smoothing the top. Refrigerate at least 4–6 hours or overnight until fully set and sliceable.
• To finish, remove the ring and acetate, warming a knife or offset spatula under hot water and drying it to smooth the sides if needed. Dust the top with cocoa and add chocolate curls. Slice with a hot, dry knife, wiping between cuts.