Sieve the flour with a pinch of salt, then pile it up on a wooden pastry board or your work surface and create a "well" in it. In the centre put the softened butter (you should take it out of the fridge half an hour beforehand), the egg yolks, sugar, grated lemon peel and vanillin. Mix the ingredients together delicately, first with a fork and then with your hands (if possible cold). Work the dough quickly to form a smooth, elastic ball of dough; sprinkle lightly with flour, put it in a bowl, cover with a tea cloth and leave to "rest" in a cool place for two hours. Put the dough in a well-buttered, round cake tin and spread and flatten it out evenly with your hands (until it's approximately 5 mm thick); all around the outside edge, leave dough to a height of roughly 2.5 cm - to hold in the marmalade and the ganache - and level it off with a pastry wheel or sharp knife. Make sure the dough adheres well to the sides by pressing with your fingers; in this way it won't fall back onto the filling while cooking. Prick the pastry bottom here and there with a fork, so that it doesn't rise while it's in the oven. Put the marmalade in a bowl and mix it with the water, to make it more liquid; this will ensure that it doesn't caramelise and keep it soft while cooking. Pour it over the dough in the cake tin and bake in a preheated oven, if possible fan-assisted, at 170 °C, for 35 minutes. In the meanwhile start preparing the chocolate ganache. Put the cream in a small saucepan and bring it to the boil (in this way, when the chocolate melts, it will become more shiny and velvety). Remove from the heat, pour the cream over the chocolate - a little at a time - and stir carefully with a whisk; continue stirring until the mixture is nice and thick. Use a spatula (ideally made of silicone) to make horizontal incisions in the ganache; this prevents the formation of bubbles on the surface when it's poured into the tart. While the ganache is still hot, pour it on top of the marmalade, in just one spot, in the centre of the tart; it will spread out evenly over the entire surface. Put it the fridge for an hour; after which remove it from the tin and place it on a plate. The tart keeps well for 4 or 5 days at room temperature.