Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans or line them with parchment paper.

In a medium saucepan, combine the unsalted butter, water, and unsweet dark cocoa powder over medium heat. Stir until smooth.

In a large mixing bowl, whisk together the all-purpose flour, light brown sugar, baking soda, and fine grain sea salt. Add espresso powder if using.

Pour the melted butter and cocoa mixture into the bowl of dry ingredients and stir gently until combined.

Add buttermilk, eggs, and vanilla extract to the batter and beat until smooth.

Divide the batter between prepared cake pans and bake for 30-35 minutes until a toothpick comes out clean.

Let the cakes cool in their pans for 10 minutes before transferring them to wire racks to cool completely.

Whip the heavy cream in a mixing bowl until stiff peaks form, then fold in semisweet chocolate chips.

In another bowl, blend cream cheese, melted white chocolate, confectioners’ sugar, and lemon juice until smooth.

Assemble the cake by layering the first cake, half the mousse, cream cheese layer, and second cake. Repeat with remaining mousse.

Heat chocolate chips, heavy cream, and sunflower oil in a saucepan over low heat until melted. Pour over the assembled cake.

Decorate the cake with chocolate buttercream, rolled wafers, and chocolate truffles.