Preheat oven to 175C/350F/160 gas.
Add eggs into a stand mixer and whisk on medium-high speed for about 5-6 mins, until eggs are pale and airy and have tripled in volume. Make sure your eggs are really triple in volume, or the recipe won’t work. Sprinkle 2 tbsp of cocoa powder on top if you like, and set the egg mixture aside.
In the meantime, pour the chocolate hazelnut spread in a large microwave-safe bowl, cover and microwave for 30 seconds.
Incorporate ⅓ of the whisked eggs into the bowl with the chocolate hazelnut spread, and very gently mix the batter with a spatula until well combined.
Fold in another ⅓ of the eggs, then gently mix until combined. Repeat with remaining ⅓ of the eggs, and again, mix until all the ingredients are combined.
Grease a 6-inch cake pan and cover the bottom and the sides with parchment paper. Pour the cake mixture into the pan and bake in the oven for 20-25 minutes.
Remove from the oven and allow to cool completely before serving.