Sift the flour in the bowl of a food processor. Add sugar, baking powder and cold butter. Pulse until the butter is all mixed with the other ingredients and the mixture is yellowish.
Transfer in a bowl or on a working surface, make a well and add egg, egg yolk and vanilla extract. Work the mixture quickly, just enough make a ball. Wrap in cling film and refrigerate for 30 minutes.
Preheat the oven to 180 C / Gas 4.
Remove 2/3 of the mixture from the fridge. Roll it out 3mm thick to a size that fits a 20 or 22cm ceramic or glass baking tin, large enough to cover bottom and sides. Cut any excess from the edges and prick the bottom. Pour in the Nutella® and spread evenly.
Remove the remaining mixture from the fridge, add to your scrapes and roll out 3mm thick. With a pastry or ravioli wheel cut out 10 to 12 strips of about 2cm width. Place on top of the filling in a crisscross pattern and pinch the ends well to seal.
Bake in the preheated oven until slightly golden, about 40 minutes. If it gets too dark, cover the top with tin foil and keep baking.