Preheat the oven to 160 C / Gas 3. Lightly grease a springform or loose-bottomed cake tin with butter.
Place dark chocolate and 225g unsalted 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.
Whisk egg whites, brown sugar and salt in a separate bowl until soft peaks form. Fold into the chocolate mixture. Pour into the cake tin and smooth the top.
Bake in the preheated oven until firm, about 20 minutes. Cool for 1 hour.
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.
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.
Meanwhile, prepare the top layer. Melt white chocolate in top of a double boiler over simmering water, about 5 minutes. Remove from heat.
Dissolve gelatine powder in water; let stand for 5 minutes. Heat in a double boiler until melted, 2 to 3 minutes. Mix into the melted chocolate.
Whisk cream until soft peaks form. Add mascarpone cheese and mix until smooth. Fold in the chocolate-gelatine mixture. Pour over the second layer. Refrigerate cake until firm, at least 3 hours.
Substitute 2 gelatine sheets for the powder, if preferred. Soak them in cold water briefly to soften before using.