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1
Preheat oven to 350F.
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2
Place eight 6-ounce (3 1/2-inch diameter) ramekins on a rimmed baking sheet, and coat with cooking spray; set aside.
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3
Stir flour, cocoa powder, sugar, baking soda, baking powder, and salt in the bowl of an electric mixer.
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4
Attach bowl to mixer fitted with the paddle attachment.
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5
Add egg, milk, oil, vanilla, and 1/4 cup water; mix on medium-low speed until smooth and combined, about 3 minutes.
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6
Divide batter evenly among prepared ramekins.
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7
Bake until a cake tester inserted into the centers comes out clean, about 20 minutes.
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8
Transfer to a wire rack; let cool completely.
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9
Run a knife around sides of cakes; unmold.
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10
The cakes can be refrigerated, wrapped in plastic, up to 1 day.
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11
Trim each cake to 1 inch high.
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12
Transfer to a baking sheet lined with parchment paper.
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13
Cut eight 10 3/4 x 4-inch strips of parchment paper.
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14
Wrap a parchment collar around base of each cake, keeping bottom flush with baking sheet.
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15
Secure each collar with tape; set aside.
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16
Transfer bittersweet-chocolate mousse to a large pastry bag fitted with a large round tip (such as Ateco #808).
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17
Pipe a 1-inch layer of mousse into each parchment collar; refrigerate until set, about 20 minutes.
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18
Repeat with milk chocolate mousse, piping on top of the bittersweet chocolate mousse.
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19
Refrigerate at least 4 hours, or overnight.
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20
Microwave semisweet chocolate until slightly warm but not melted, about 30 seconds.
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21
Scrape at a 45-degree angle with a vegetable peeler, forming curls.
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22
Before serving cakes, remove parchment collars, and garnish with chocolate curls.
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23
Put 1 2/3 cups cream into the bowl of an electric mixer fitted with the whisk attachment; beat on medium-high speed until soft peaks form, about 3 1/2 minutes.
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24
Transfer to a bowl; refrigerate 1 hour.
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25
Put 4 egg yolks into the clean bowl of the mixer fitted with the clean whisk attachment; beat on high speed until pale and frothy, 4 to 5 minutes.
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26
Meanwhile, bring 1/4 cup sugar, 2 tablespoons corn syrup, and 2 tablespoons water to a rolling boil in a small, heavy saucepan over high heat.
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27
Cook until clear, large bubbles form, about 1 minute.
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28
Reduce mixer speed to medium-low.
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29
Carefully pour hot syrup down side of bowl.
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30
Raise speed to medium-high.
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31
Mix until slightly thickened, about 5 minutes.
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32
Stir in bittersweet chocolate, 1 teaspoon vanilla, and a pinch of salt with a rubber spatula.
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33
Add one-third of bittersweet-chocolate mixture to whipped cream; whisk to combine.
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34
Add remaining mixture, whisking until completely combined.
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35
Press through a large-mesh sieve into a large bowl; discard any solids.
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36
Repeat entire recipe, substituting milk chocolate for the bittersweet.
