For the shortcrust pastry, mix the egg and yolk with the sugar and butter. Mix the flour with the baking powder, salt and lemon zest and gradually knead with the remaining ingredients to form a dough. Wrap the dough in foil and let it rest in the refrigerator for about 30 minutes.
For the lemon cream, bring the milk with the lemon zest to the boil and let it steep for 5 minutes. In a bowl, stir the egg yolks and sugar with a whisk. Add the sifted cornstarch.
Remove the lemon peel from the milk, add the egg mixture while stirring and bring to a boil over low heat, stirring constantly, until the mixture thickens. Then let it cool, stir in the lemon juice and cover with cling film.
Take the shortcrust pastry out of the refrigerator. Set aside some of the batter that will be used for the strips. Roll out the dough and place in a greased, 26 mm tart pan. Prick the bottom lightly with a fork.
Pour the lemon cream on the batter. Roll out the remaining shortcrust pastry and cut into strips. For decoration, place the shortcrust pastry strips in a diamond shape over the lemon cream and brush with the whisked egg yolk.
Bake the tart in a preheated oven at 180 ° C top / bottom heat for about 45 - 50 minutes. When the surface is golden brown, the crostata is ready.
Let the tart cool completely, then place in the refrigerator and let rest for a few hours. Dust the crostata al lime with powdered sugar.