In a large saucepan, stir together the sugar, chocolate, butter, cocoa powder, Nutella, espresso powder, baking soda, and 2 cups (500ml) water. Heat oven medium for 3 minutes and stir until the sugar dissolves. Increase heat to high, bring to a boil, then remove from heat. Pour mixture into a large heatproof mixing bowl and set aside to cool for about 20 minutes.

Preheat the oven to 350°F. Generously grease and line two 8-inch cake pans with parchment rounds.

Once cool, stir in the eggs one at a time. Gently stir in the flour and salt. Divide the batter between the prepared pans. Bake for 40 minutes, or until a cake tester comes out clean. Cool for 10 minutes then flip the cakes out onto cooling racks to finish cooling.

In the bowl of a stand mixer fitted with the paddle attachment, mix together the sugar and butter. Mix on low speed until well blended and then increase the speed to medium and beat for another 3 minutes. Add the cocoa powder, Nutella, and salt and beat on medium for 1 minute, or until smooth.

Use a large serrated knife or cake leveler to level the top of each cake, then to cut each cake layer in half. Place 1 cake layer on a cake platter or turntable. Spread with a fourth of the buttercream and top with another cake layer. Repeat with all four cake layers. Spread the remaining frosting evenly over the top of the cake. Decorate with edible flowers, seasonal candy, Ferrero Rocher, or whatever your heart desires.