Let’s see how to prepare the crumble.

In a bowl, pour the flour, add the sugar, cold butter cut into small pieces, baking powder, vanillin, a pinch of salt, the grated zest of 1 lemon, preferably organic. With your hands, start breaking down the butter until it becomes very small, it should look floury. At this point, add the eggs, and still with your hands, break down the eggs with the whole mixture; it should incorporate but not compact because it should remain all crumbly. If there are some large pieces, make them smaller. If the Nutella is nice and soft, you don’t have to do anything, but in the colder months, it tends to harden, so you need to take the jar of Nutella and put it in a bain-marie to make it softer, so it will be easier to spread. Take the baking pan, line the base with parchment paper, and fill the bottom of the pan with the prepared mixture, compacting slightly with your hands. When the Nutella is ready, pour it inside, spreading it while leaving the edges free; cover with the remaining mixture without compacting, so that the crumbs are visible, and cover the edges as well. Bake in the hot oven and cook at 350°F for about 15 minutes or until the desired golden color. When it’s ready, let it cool slightly before removing it from the mold.