1. Chocolate Cake Preparation:

• Preheat oven to 170°C. Grease and dust two 6-inch baking tins with flour or cocoa powder.
• Boil water and whisk hot water with cocoa powder until dissolved. Set aside to cool slightly.
• Beat butter and brown sugar until light and fluffy.
• In a bowl, sift together flour, baking powder, bicarbonate of soda, and salt. Set aside.
• Mix in egg and egg yolk to the butter mixture. Add melted chocolate and mix well.
• Alternately add flour mixture and milk in three parts, starting and ending with flour.
• Stir in cocoa mixture and divide batter equally between the prepared tins.
• Bake for 25-30 minutes. Cool in tins for 10 minutes, then invert onto a rack to cool completely. Chill in the fridge.

2. Chocolate Cheesecake:

• Grease and line a 6-inch springform tin with parchment paper. Wrap in foil.
• Melt chocolate and cool.
• Beat cream cheese and condensed milk until smooth. Gradually beat in sugar, salt, eggs, and vanilla extract. Mix in melted chocolate.
• Place springform tin in a roasting tray, add hot water to the tray.
• Bake for 60 minutes. Turn off oven, leave cheesecake inside for an hour. Cool completely, then freeze.

3. Nutella Frosting:

• Beat butter and vegetable shortening until creamy.
• Gradually add sifted icing sugar.
• Add Nutella, mascarpone cheese, whipping cream, and vanilla extract. Beat until light and fluffy.

4. Chocolate Ganache:

• Melt chocolate and cream in the microwave in 30-second bursts, stirring until smooth.
• Cool ganache for 10 minutes. Adjust consistency with vegetable oil if needed.

5. Assembling the Cake:

• Level each cake layer.
• Spread a thin layer of frosting on a cake board. Place one cake layer down.
• Pipe Nutella frosting on the cake. Add the cheesecake layer, more frosting, and then the second cake layer.
• Crumb coat the cake and chill for 30 minutes.
• Apply a final coat of Nutella frosting. Chill for an hour.
• Pour ganache over the chilled cake.
• Pipe swirls of Nutella frosting on top. Garnish as desired.