Preheat your oven to 325 F (160 C) degrees. Line a 8x8 inch baking dish with parchment paper.
Separate the eggs and add the egg whites to the bowl of your mixer. Mix until egg whites are stiff. Place the egg whites in a bowl and set aside.
Add the egg yolks and icing sugar the the bowl of your mixer and mix together until mixture becomes like a cream and is light pale yellow in color.
Add the Nutella, melted butter, vanilla extract, salt, cocoa powder and continue mixing for another minute until well incorporated.
With the mixer on low, start adding flour to the mixer, one tablespoon at a time, until all the flour has been incorporated into the mixture.
With the mixer still on low, slowly start adding the milk and beat until everything is well mixed together. The mixture should be very thin, almost like a crepe batter.
Start gently folding in the egg whites, a third at a time using a spatula. Repeat until all the egg whites have been folded in. They do not have to be completely mixed in, it's OK to still see foamy bits floating around the top of the batter.
Pour the batter into the prepared baking dish and bake for 40 to 70 minutes or until the top is browned. The cake should be firm to touch, and should not wiggle.
Let cool completely, then place in the fridge for at least 2 hours.
Sprinkle some icing sugar before slicing into it.