Preheat oven to 180 °C (350 °F). Grease or butter individual fluted tart tins or muffin cups.

In a bowl, whisk flour, sugar, and salt. Add cold butter and rub or use a pastry cutter until mixture resembles coarse crumbs.

Add the egg and egg yolk, mixing until dough begins to come together; add milk as needed. Form into a disk, wrap, and chill for 30 minutes.

Meanwhile, prepare custard: whisk egg yolk, sugar, and cornstarch. Heat milk (with lemon zest if using) until hot but not boiling. Gradually whisk hot milk into egg mixture, then return to pan and cook over medium-low heat, stirring until thickened. Remove from heat, stir in vanilla, then cool and chill.

Roll chilled dough to about 3 mm thickness. Cut circles to line the bottom and sides of each tart tin, leaving surplus to fold over. Press in gently.

Spoon chilled custard into each pastry shell nearly to the top.

Fold the dough edges over the custard, partially covering. Brush lightly with beaten egg.

Bake for about 20–25 minutes or until crust is golden and filling just set. Cool slightly before unmolding.

Serve warm or at room temperature; dust with powdered sugar if desired.