Preheat the oven to 180°C / 350°F.

Line a 20 cm cake pan with parchment paper.

Separate the eggs into yolks and whites.

In a bowl, whisk the egg yolks with the sugar until pale and creamy.

Add the flour, baking powder, and oil. Mix until combined.

In a clean bowl, beat the egg whites until stiff peaks form.

Gently fold the egg whites into the yolk mixture.

Pour the batter into the prepared pan and smooth the top.

Bake for about 10 minutes, or until lightly golden and springy to the touch.

Let the sponge cool completely, then slice it into 2 even layers.

In a bowl, combine the mascarpone, powdered sugar, and Nutella.

Beat briefly until smooth.

In another bowl, whip the heavy cream to firm peaks.

Fold the whipped cream gently into the Nutella mixture.

Divide the filling into 2 equal portions.

Place the first cake layer on a serving plate.

Brush with 60 ml milk.

Spread over half of the filling.

Add the second cake layer and brush with the remaining 60 ml milk.

Spread the rest of the filling on top.

Refrigerate for at least 2 hours.

Place the chopped chocolate in a bowl.

Heat the heavy cream until just beginning to boil.

Pour the hot cream over the chocolate and let sit for 1 minute.

Stir until smooth and glossy.

Let the ganache cool for 5 to 10 minutes, until slightly thickened but still pourable.

Pour the ganache over the chilled cake.

Decorate with Kinder chocolates, chocolate curls, or chopped hazelnuts if using.

Chill for another 1 to 2 hours, or overnight, before serving.