Chocolate Cake:

1. Boil the milk with sugar until fully dissolved. Mix in the cocoa powder to form a smooth chocolate mixture.
2. Add the egg, oil, and vinegar to the chocolate mixture. Make sure the mixture isn’t too hot when adding the egg to avoid scrambling.
3. In a separate bowl, combine the dry ingredients: flour, baking powder, baking soda, salt, and cinnamon.
4. Gently fold the wet mixture into the dry ingredients until just combined. Do not overmix to keep the cake moist and soft.
5. Pour the batter into a lined 17 cm x 6 cm (7 x 2.5 inch) springform pan.
6. Bake at 180°C (350°F) for 25–30 minutes or until a wooden skewer comes out clean. Let the cake cool completely.

Cheesecake Filling:

1. In a bowl, whip the cream with sugar until soft peaks form. This adds air to the cream and helps the filling hold its structure.
2. In another bowl, work the cream cheese with Nutella using a spatula until smooth.
3. Gently fold the whipped cream into the Nutella–cream cheese mixture to create a light, creamy filling.

Assemble the Cake:

1. Spread all of the cheesecake filling over the cooled chocolate cake.
2. Using a spoon, make a small well in the center and fill it with Nutella. Cover the Nutella with the surrounding cheesecake filling so it is completely enclosed.
3. Refrigerate for at least 8 hours, or overnight for best results.

Decorate:

1. Melt the Nutella and mix with chopped hazelnuts.
2. Place a small plate and a cooling rack under the cake, then pour the Nutella-hazelnut mixture over the top, spreading evenly to cover the entire cake. Let set for 10 minutes.
3. Slice and enjoy!