When I made these the first time, I put them on the table after supper and I have never seen a plateful of anything go down so fast. Naturally, I have been making them a lot ever since. As the ingredients—yes, all of them—are among my kitchen basics, I know I always have the wherewithal to make these, unplanned. Having said that, this is one of those recipes which is actually better the following day, not that it's likely you'll get a chance to find out.
Preheat your oven to 350 degree F. Line the base and sides of your cake tin with parchment paper.
Break the eggs into a mixing bowl, then add the salt and beat, using a freestanding mixer with the whisk attachment or electric handheld whisk, until they have more than doubled in size and are pale, aerated and mousselike; this will take about 5 minutes.
Measure the Nutella into a microwave-safe jug, filling it up to the 1 cup mark, and microwave for 1 minute. Or you can soften it in a heatproof bowl over (but not in) a saucepan of simmering water for 3-4 minutes, stirring occasionally, until warm and slightly runny.
Stir the heated Nutella, then pour it in a continuous thin stream onto the eggs, whisking as you go, until all the Nutella is combined. This will knock the volume outof the eggs quite a bit, but don't worry about it.
Pour this mixture into your prepared cake tin and bake for 17-20 minutes, by which time the top will be dry, and the middle tender but set in a slightly jellied fashion.
Leave to cool completely in the tin; the mixture will shrink away from the sides a little as it does. Once cool, cut it into 16 squares, arrange them on a plate and dust with a little icing sugar pressed through a fine tea strainer or sieve.
Store in an airtight container in a cool place for up to 5 days or in fridge for up to 1 week. In hot weather, store in fridge.
Stack the squares in an airtight container with baking parchment in between the layers. Freeze for up to 3 months. To defrost, put squares on a wire rack and leave at room temperature for about 1 hour, or defrost overnight in fridge.