Procedure

Beat the butter with the planetary mixer until it's soft. Add the sieved flour and mix well. Add the salt and vanilla.
Work in the egg and the sieved icing sugar. When the dough is smooth and even, cover it with cling film and leave it in the fridge for two hours (for better results, you are advised to prepare the dough the day before).
Divide the dough in half and roll each piece out to a thickness of about ½cm (on two separate baking trays).
Cut ten 7x7cm squares out of the first piece. Cover them with cling film and place in the fridge.
Cut narrow 8mm strips from the second piece and place them in the fridge
When working with shortcrust pastry, it's always a good idea to keep it at fridge temperature to make the job easier.
Line a baking tray with greaseproof paper and arrange the dough strips in a lattice pattern, as if you were making a large tart. Leave to cool thoroughly, then use a knife to cut the pattern into ten 7x7cm squares.
Bake both the squares and the lattices on separate trays in the oven at 175°C for about 8 minutes, until golden.
Leave to cool, then spread each square with 15g Nutella® and place a pastry lattice on top.