Mix sugar, flour, and salt in a bowl and add the butter. Crumble the butter and press the dough together - add 1/2 tbsp of water if the dough is difficult to bring together. Line a small baking pan with baking parchment and press the dough into it. It should be 0.5 cm thick. Prick the base with a fork. Bake the base at 180 degrees Celsius (356 degrees Fahrenheit) for about 15 minutes, until it is slightly golden at the edges. Whisk eggs and sugar together to form a thick, white and frothy mass. Add Nutella and flour and whisk it in. Spread the mixture over the pre-baked base. Return the cake to the oven and bake it for about 25 minutes. When the surface has "puffed" up and is just baked, remove the cake. It must not be overbaked. Cool the cake completely before cutting it into suitable pieces.