Start by making the crust: quickly mix the butter with the sugar, then add the eggs, followed by the flour, baking powder, lemon zest, and salt. Once the dough is smooth and even, shape it into a ball, place it in a bowl, and cover with plastic wrap. Let it rest in the refrigerator for about 30 minutes.

Meanwhile, prepare the pastry cream. Heat the milk with the seeds from the vanilla bean. In a separate bowl, whisk the egg yolks with the sugar, then gradually add the flour. Combine this mixture with the milk and whisk well. Continue stirring with a wooden spoon until the cream thickens completely.

Let the pastry cream cool and take the dough out of the fridge. Take 3/4 of the dough and roll it out on a lightly floured surface, reserving the rest for the top. Butter and flour a 28 cm (11-inch) tart pan and press the dough into the pan, making sure it goes up the sides. Spread a layer of blackberry jam to your liking. Make sure the pastry cream is cool, then pour it into the pan and smooth it out evenly.

Roll out the remaining dough and cover the tart completely. Trim any excess dough and gently press the edges together to seal. Bake in a preheated oven at 350°F (180°C) for 30–35 minutes. Then open the oven door and let the crostata dry out a bit in the warm oven. Remove from the oven and let it cool completely before removing from the pan. You can dust with powdered sugar if you like.