Cake

• 1 cup unsalted butter, cubed
• 1 cup water
• 1/3 cup unsweetened dark cocoa powder
• 2 cups all-purpose flour
• 1 1/2 cups packed light brown sugar
• 2 teaspoons espresso powder (optional)
• 1 teaspoon baking soda
• 1/2 teaspoon fine grain sea salt
• 1/2 cup buttermilk, at room temperature
• 2 large eggs, at room temperature
• 1 teaspoon vanilla extract

Chocolate Mousse Layer

• 1 cup heavy cream, divided
• 1/2 cup semisweet chocolate chips or finely chopped semisweet chocolate

Vanilla Cream Cheese Layer

• 4 oz. cream cheese
• 3 oz. white chocolate, melted and cooled slightly
• 1/3 cup unsalted butter, at room temperature
• 1/2 tablespoon lemon juice
• 1/3 cup confectioners’ sugar
• 1 teaspoon vanilla extract

Marbled Ganache Topping

• 1 cup semisweet chocolate chips or finely chopped semisweet chocolate
• 1/2 cup heavy cream
• 1 tablespoon sunflower oil or vegetable oil
• 1/2 cup confectioners’ sugar
• 1 tablespoon milk, plus more if needed

Chocolate Buttercream and Décor

• 3 tablespoons unsalted butter, at room temperature
• 1/2 cup confectioners’ sugar
• 2 tablespoons unsweetened cocoa powder
• Milk or cream to thin
• 1 rolled cookie wafer, such as Pirouette
• 3 fancy chocolate truffles, store-bought