Prepare the lemon cream filling: In a medium saucepan, whisk together 1 egg, 2 tablespoons cornstarch, 3 tablespoons sugar, and 250 ml milk.

Cook over medium heat, stirring continuously until the mixture thickens and turns into a smooth, creamy texture.

Remove from heat and stir in 40 ml lemon juice and optional lemon zest for extra zing.

Cool the filling: Transfer the lemon cream to a bowl, cover with plastic wrap (press the wrap directly onto the surface of the cream to prevent a skin from forming), and refrigerate for about 30 minutes until cooled.

Prepare the puff pastry: Roll out the puff pastry sheets and cut them into squares or circles, depending on your preferred tart shape.

Place the cut pastry pieces into a muffin tin or tart molds, pressing them lightly into place to form a tart shell.

Brush with milk: Lightly brush the top edges of the puff pastry with milk to give them a golden color when baked.

Bake the puff pastry: Preheat the oven to 180°C (350°F).

Bake the puff pastry shells for about 20 minutes or until golden brown and puffed up. Remove from the oven and let cool.

Assemble the tarts: Once the pastry shells and lemon cream are both cool, spoon the chilled lemon cream into the puff pastry shells.

Dust with powdered sugar: Lightly dust the tops of the tarts with powdered sugar before serving.