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1.
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Preheat the oven to 350F.
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Butter a 9-inch round cake pan.
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Dust the pan with flour and tap out the excess.
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2.
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Whisk together the flour and cocoa powder in a medium bowl; set aside.
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3.
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Fill a pot one-third full with water and bring to a simmer.
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Combine the eggs and sugar in the bowl of an electric mixer.
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Place the bowl over the simmering water and whisk until the mixture is warm to the touch.
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Transfer the bowl to the mixer stand and, using the whisk attachment, beat on medium speed until the mixture triples in volume and forms a thick ribbon when the whisk is lifted, about 8 minutes.
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Using a large rubber spatula, gently fold in the flour mixture.
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Fold in the melted butter.
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Scrape the batter into the pan and smooth the top.
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4.
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Bake the cake for 20 to 25 minutes, until it has pulled away from the side of the pan and the top springs back when lightly touched.
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Cool the cake in the pan for 10 minutes.
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Unmold the cake on a rack and cool completely, right side up.
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5.
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In the bowl of an electric mixer, using the whisk attachment, beat 2 1/2 cups (580 grams) of the heavy cream to soft peaks.
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Scrape half of the whipped cream into a medium bowl.
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Cover both bowls and refrigerate.
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6.
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Put the milk and dark chocolate in two separate medium bowls.
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Bring the remaining 1 cup (232 grams) cream to a boil in a small saucepan.
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Pour 1/2 cup (116 grams) of the hot cream over the milk chocolate and the remaining 1/2 cup (116 grams) over the bittersweet chocolate.
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Gently whisk the milk chocolate until it is completely melted and the mixture is smooth.
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Whisk the dark chocolate until smooth.
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7.
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Let the chocolate mixtures cool for 10 minutes.
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Fold the milk chocolate mixture into one bowl of whipped cream.
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Fold the dark chocolate into the remaining whipped cream.
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8.
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Combine 1/4 cup (59 grams) water and the sugar in a small saucepan.
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Bring to a boil over medium-high heat, stirring to dissolve the sugar.
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Let the syrup cool to room temperature.
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9.
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Using a serrated knife, slice the cake horizontally in half.
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Cup a strip of parchment paper to line the inside of a 9x4-inch cake ring.
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Line the mold with the strip and place it on a serving plate.
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Place a 9-inch cardboard cake round in the bottom of the cake ring.
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Place the bottom cake round, cut side up, in the ring.
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Brush the cake generously with the sugar syrup.
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Scrape the dark chocolate mousse onto the cake and smooth it into an even layer.
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Top with the second cake round, cut side down.
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Brush the cake generously with syrup.
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Scrape the milk chocolate mousse onto it, smoothing it into an even layer.
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Chill the cake for at least 2 hours and up to 24 hours.
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10.
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To unmold the cake, lift it off the plate, holding it with your palm against the cardboard round.
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Using a hairdryer or warm damp towel, briefly warm the cake ring mold.
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Lift off the ring mold and peel off the parchment strip.
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Allow the cake to stand at room temperature for 30 minutes before serving.
