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1
Make the cake:
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Adjust the oven rack to the middle of the oven and heat to 350u00b0F. Lightly grease and flour 1 8-inch springform pan. Tap out any excess flour.
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Place the chocolate in a small heatproof bowl. Bring water to a boil and pour it over the chocolate. Stir with a silicone spatula until the chocolate is melted and smooth, about 2 minutes. Add the coffee granules and cocoa powder. Whisk until smooth. Add 1/4 cup of the sugar and stir until thick and glossy, about 2 minutes.
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In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and remaining sugar together until light and fluffy. Beat in the eggs and egg yolks, one at a time, mixing well after each addition. Beat in the vanilla. Add the chocolate mixture and mix until thoroughly combined.
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In a small bowl, combine the flour, baking soda, and salt. Stir with a whisk. Add half of the flour mixture to the chocolate mixture, followed by the buttermilk, mixing until incorporated. Add remaining flour mixture and mix until the batter is thoroughly combined.
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Spread the batter into the bottom of the prepared pan, smoothing it to the edges with a spatula. Bake for 25 to 30 minutes or until a toothpick inserted into the center comes out clean. Cool to room temperature in the pan on a wire rack.
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Make the mousse:
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In a small bowl, sprinkle the gelatin over water, let stand for 5 minutes. Place chocolate and sugar in a medium bowl. Bring 1/2 cup of the cream to a simmer in a small saucepan over medium-high heat. Remove from heat; add the softened gelatin and stir until fully dissolved. Pour the cream over the chocolate and whisk until chocolate is melted and mixture is smooth. Cool to room temperature, stirring occasionally. The mixture will thicken slightly.
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In the bowl a stand mixer fitted with the whisk attachment, whip the remaining 1 cup cream at medium speed until it begins to thicken, about 30 seconds. Increase the speed to high and whip until soft peaks form, about 30 to 60 seconds. Use a whisk to mix in 1/3 of the whipped cream into the chocolate mixture. Use a rubber spatula to fold the remaining whipped cream into the chocolate mixture until no streaks appear. Spoon the chocolate mousse into the pan over the chocolate cake. Smooth the top with an offset spatula. Return the cake to the refrigerator and chill until set, about 2 hours.
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Make the ganache:
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Heat the cream and butter in a medium saucepan over medium-high heat. Remove from heat just before it begins to boil. Place the chocolate in a medium heatproof bowl and pour the hot cream over the chocolate. Stir until chocolate is melted and smooth. Cool to room temperature. Spread the ganache over the top of the mousse layer and refrigerate until set. About 45 minutes.
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Run a knife along the outer edge of the cake to loosen it from the pan and gently remove from the pan. Serve with whipped cream or fresh berries.
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Notes.
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Total time does not reflect the time needed for the mousse and ganache to set. Please allow 3 extra hours for this.
