Start with the crust: Mix the sugar and softened butter together until well combined. Add the eggs and whisk by hand. Finally, add the flour, lemon zest, and a pinch of salt. Knead the dough until it forms a ball, wrap it in plastic wrap, and refrigerate for 30 minutes to 1 hour.

For the pastry cream: Whisk the egg yolks and sugar together by hand, then add the sifted flour, vanilla, and lemon zest, mixing again. In a separate saucepan, heat the milk. When the milk is almost boiling, add a few tablespoons to the egg mixture and stir until smooth. Pour the egg mixture into the saucepan with the remaining milk and stir over heat until the cream thickens. Cover the cream with plastic wrap directly on the surface and let it cool.

After chilling, roll out the dough and place it in a 10- to 11-inch (26-28 cm) tart pan, pricking the bottom with a fork and forming the edges. Reserve a small portion of the dough to make decorative strips for the top. Fill the crust with the pastry cream using a spatula, sprinkle with dark chocolate shavings, and arrange the dough strips in a lattice pattern on top. Bake at 350°F (180°C) for 30 minutes. Let the crostata cool before serving.