Preheat the oven to 175°C (350°F / gas 160) and prepare a 6-inch cake pan by greasing it with spray oil and lining the bottom and sides with parchment paper.
Place the 5 large eggs in a stand mixer fitted with the whisk (or use an electric hand mixer) and beat on medium-high speed for 5–6 minutes, until the eggs are pale, very airy, and roughly tripled in volume.
If using cocoa powder, sprinkle the 2 tablespoons over the whipped eggs and set the bowl aside.
Warm the 250 g chocolate-hazelnut spread in a large microwave-safe bowl for 30 seconds, then stir until smooth and slightly loosened.
Fold about one-third of the whipped eggs into the spread gently with a silicone spatula to loosen the chocolate mixture.
Fold in the second third of the whipped eggs gently until combined, then fold in the remaining third using gentle motions to preserve the airiness.
Pour the batter into the prepared pan, smoothing the top gently, and bake for 20–25 minutes until the top is set; for a slightly gooey center aim for about 22 minutes.
Remove the cake from the oven and allow it to cool completely in the pan before unmolding and serving.