In a bowl, whisk together all dough ingredients except flour. Then stir in flour with a spoon.
Move dough to a flat surface and knead by hand until you have an elastic dough that no longer sticks to your hands (usually 280g is enough, but if it's still sticky add more flour in 10g increments). Cover the dough with the bowl and let it rest for 15 minutes. In the meantime, butter the tart pan (skip if pan is non-stick). Place the unopened jar of Nutella in a bowl of hot water so the Nutella will soften.
Preheat oven to 170°c.
Roll the dough out on a lightly floured surface to a thickness of about 5mm. Gently roll the dough around the rolling pin and then unroll onto the pan. Now roll the rolling pin over the pan to cut off the excess dough. Pour the Nutella into the tart base and even it out with a spatula or the back of a spoon.
Roll the excess out into a rough rectangle. Aim for a thickness of 4mm. Cut six strips and gently place them on the tart in order the create the lattice.
Bake for 20 minutes, then cover with aluminium foil and bake for a further 15 minutes.