Finely grate the zest from the lemon. Reserve half for the pastry and half for the custard.

Whisk 100 g granulated sugar, olive oil, 1 whole egg, and 1 egg yolk until smooth. Add half of the lemon zest.

Add flour, salt, and baking powder to the wet mixture. Mix just until a smooth dough forms.

Wrap the dough in plastic wrap. Refrigerate it for 30 minutes.

Set aside 50 ml whole milk and whisk it with cornstarch until lump-free. Keep the mixture nearby.

Heat the remaining 200 ml whole milk with the remaining lemon zest until steaming. Do not let it boil vigorously.

Whisk 50 g granulated sugar with 2 egg yolks until pale. Slowly whisk in some hot milk to temper the yolks.

Return the egg mixture to the saucepan and stir continuously over medium-low heat. Add the cornstarch milk mixture.

Cook the custard while stirring until thick and smooth. Cover its surface with plastic wrap and let it cool.

Heat heavy whipping cream until hot but not boiling. Keep it warm for the caramel.

Cook 50 g granulated sugar with water in a saucepan until it becomes golden caramel. Carefully stir in the hot cream.

Stir the toffee over low heat until smooth and slightly thickened. Cover its surface and let it cool.

Preheat the oven to 170°C / 340°F. Grease the tart pan if needed.

Roll out about two-thirds of the chilled dough. Line the base and sides of the tart pan with it.

Spread the cooled custard evenly in the pastry shell. Spoon the cooled toffee in an even layer over the custard.

Roll out the remaining dough and place it over the filling. Seal the pastry edges.

Bake the tart for about 45 minutes until golden. Cool completely before removing it from the pan.

Dust the cooled tart with icing sugar. Slice into 8 portions and serve.