Step 1
Preheat the oven to 325 degrees F (165 degrees C). Lightly grease a springform pan with butter.
Step 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.
Step 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.
Step 4
Bake in the preheated oven until firm about 20 minutes. Cool for 1 hour.
Step 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.
Step 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.
Step 7
Meanwhile prepare the top layer. Melt white chocolate in top of a double boiler over simmering water about 5 minutes. Remove from heat.
Step 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.
Step 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.