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1
Preheat the oven to 325 degrees F (165 degrees C). Lightly grease a springform pan with butter.
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2
Place dark chocolate and butter in top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Remove from heat. Stir in coffee and let cool slightly, about 5 minutes. Whisk egg yolks into the chocolate.
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3
Whisk egg whites, brown sugar, and salt in a separate bowl until peaks are the consistency of firm snow. Fold into the chocolate mixture. Pour into the pan and smooth the top.
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4
Bake in the preheated oven until firm, about 20 minutes. Cool for 1 hour.
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5
Prepare the second layer while the first layer is cooling. Mix water and cocoa powder together in a bowl. Melt dark chocolate in top of a double boiler over simmering water, about 5 minutes. Remove from heat.
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6
Whisk cream, mascarpone cheese, sugar, and salt together in a bowl. Add cocoa powder mixture and melted chocolate. Pour over the first layer. Refrigerate until set, at least 30 minutes.
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7
Meanwhile, prepare the top layer. Melt white chocolate in top of a double boiler over simmering water, about 5 minutes. Remove from heat.
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8
Dissolve gelatin powder in water; let stand for 5 minutes. Heat in a double boiler until melted, 2 to 3 minutes. Mix into the melted chocolate.
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9
Whisk cream until soft peaks form. Add mascarpone cheese and mix until smooth. Fold in the chocolate-gelatin mixture. Pour over the second layer. Refrigerate cake until firm, at least 3 hours.
