Mix the flour with the sugar, the salt and the baking powder. Add the diced butter and rub all the ingredients with your fingertips as to make soft crumbles, just as grated Parmesan cheese.
Beat a whole egg and two yolks in a bowl, then add them to the crumbles and keep rubbing the ingredients with your fingertips until you have a nice and smooth ball of dough. Flatten the dough ball with your hands, wrap it in cling film and let it rest in the fridge for at least one hour.
Preheat oven to 180°C.
Divide the dough into two equal parts: line a 24 cm buttered round baking pan with half of the shortcrust dough, then spread half of the Nutella on the pie and sprinkle with the chopped walnuts.
Roll out the second part of shortcrust pastry in a disk big enough to cover the cake. Press the edges to seal the pie and bake in the preheated oven for 40 minutes.
Whip the egg whites until soft peaks form then gradually add 4 tablespoons of sugar, keeping on whipping the egg whites until the meringue is firm and glossy.
After 40 minutes of baking, when the cake is golden, remove it from the oven and spoon the meringue over the cake, using a spatula or the back of a spoon to shape it and create some puffs of meringue.
Stir quickly the leftover Nutella in the jar to heat it up, then drizzle it on the meringue and mix them gently with a fork to form crazy doodles.
Bake again the cake for 2 minutes, and as soon as the meringue gets firm on the surface and slightly golden, remove from the oven and let it cool completely before you slice it.