For the filling: start by preparing the pastry cream bring the milk to the boil with the vanilla pod and lemon zest in it, then leave it to cool. In the meanwhile, beat the egg yolks with the sugar until thick and fluffy. Stir in the sifted flour, a little at a time, making sure it is well amalgamated. Then very slowly pour in the now cold milk (from which you have removed the vanilla pod and lemon zest). Put the mixture on the stove over a very low heat and, stirring continuously with a wooden spoon, bring it almost to the boil so that the pastry cream thickens, then remove it from the heat. If any lumps should have formed while cooking, beat with a whisk to make it smooth. Now leave it to cool, stirring occasionally to prevent a “skin” forming on the surface. Mix the ricotta, which you have pressed through a coarse meshed sieve, with the sugar and leave to rest for 30 to 40 minutes. Then wrap it in a cloth and squeeze delicately to get as much liquid out as possible. Mix the ricotta with an egg yolk and gradually add the cooled pastry cream, amalgamating everything thoroughly. Now move on to the pastry: mix the flour with the potato starch, pile it up in a heap on your work surface and form a "well" in the centre. Break the egg into it, add the other yolk, the sugar, the butter (taken out of the fridge half an hour before and cut into small pieces), cinnamon and salt. Mix everything together well and form a ball of dough. Grease a round tart tin with butter (the ideal thing would be a springform tin with a removeable bottom). Place the ball of dough in the centre and gradually flatten it out with your fingers to line the bottom and around the side of the tin. Pour in the filling, level it by tapping the tin on the work surface a few times, and fold a little dough in over it all around the edge. Bake in a preheated oven, at 170 °C, for 30-40 minutes. Then take it out and leave it to cool. The tart is delicious when still warm, but also cold, the following day, as a tasty nutritious breakfast.