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1
Make the cake: Prepare the cake mix as the label directs for a 9-by-13-inch cake.
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2
Bake; cool slightly in the pan, then invert onto a rack to cool completely.
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3
Clean out the cake pan and line it with plastic wrap.
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4
Cut the cake in half lengthwise, then in thirds crosswise to make 6 rectangles.
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5
Using a serrated knife, slice each rectangle in half to make two layers.
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6
Arrange half of the pieces snugly in the cake pan.
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7
Crumble the remaining pieces and press tightly into 6 small balls; arrange 1 cake ball in the center of each cake rectangle in the pan.
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8
Make the mousse: Heat the chocolate, butter, coffee and 1/4 cup water in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water), stirring, until melted.
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9
Remove the bowl from the pan; stir until cool.
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10
Reserve the simmering water.
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11
Fill a large bowl with ice water.
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12
Whisk the egg yolks, 1 1/2 cups sugar and 2 tablespoons water in a separate large heatproof bowl.
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13
Set the bowl over the saucepan of simmering water and whisk until pale yellow, 6 to 8 minutes.
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14
Add the chocolate-coffee mixture and the vanilla and whisk until combined, about 2 minutes.
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15
Remove the bowl from the pan and set in the bowl of ice water; whisk until slightly cool but not thick, about 4 minutes.
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16
Beat the egg whites and salt in a bowl with a mixer until foamy.
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17
Add 2 tablespoons sugar and beat until almost stiff; gently fold into the chocolate-yolk mixture to make a dark chocolate mousse.
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18
Spread 5 cups over the cake and cake balls.
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19
Freeze until firm on top, about 30 minutes.
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20
Beat the heavy cream and the remaining 4 tablespoons sugar until soft peaks form; fold into the remaining mousse.
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21
Remove the cake from the freezer; spread with the light chocolate mousse, cover with plastic wrap and freeze until firm, at least 6 hours or overnight.
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22
Make the chocolate shell: Stir the chocolate, heavy cream and 4 tablespoons corn syrup in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water) until melted.
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23
A spoonful at a time, spread half of the shell mixture over the frozen mousse.
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24
Return to the freezer.
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25
Make the chocolate sauce: Add the remaining 1 tablespoon corn syrup and the butter to the remaining chocolate shell mixture.
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26
Microwave 30 seconds, then stir until glossy.
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27
To serve the cake, beat the heavy cream with a mixer until foamy.
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28
Add the sugar and beat until soft peaks form.
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29
Remove the cake from the freezer; invert onto a baking sheet and unmold, then invert again onto a platter, chocolate shell-side up.
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30
Cut the cake in half lengthwise using a warm knife, then cut into thirds crosswise to make 6 rectangles; cut each rectangle in half diagonally to make 2 triangles.
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31
Top each triangle with whipped cream, ice cream and the prepared chocolate sauce.
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32
Photograph by Kang Kim
