Prepare the Crust:
Preheat your oven to 350°F (175°C). In a mixing bowl, combine the flour, butter, sugar, and egg. Mix until it forms a dough. Roll the dough out on a lightly floured surface and press it into the base of a 9-inch tart pan. Prick the base of the crust with a fork to prevent bubbling. Bake for 10-12 minutes until lightly golden, then cool.

Make the Filling:
In a saucepan, combine the milk, sugar, and cornstarch. Whisk until smooth. Heat over medium heat, stirring constantly, until the mixture thickens (5-7 minutes). In a separate bowl, whisk eggs and vanilla extract. Gradually add a spoonful of the hot milk mixture to the eggs to temper them. Then, pour the egg mixture back into the saucepan and cook for another 2-3 minutes. Remove from heat and stir in the citrus zest.

Assemble the Melktert:
Pour the cooled filling into the pre-baked crust. Smooth it out with a spatula. Allow the tart to cool to room temperature, then refrigerate for at least 3 hours or until the filling is fully set.

Serve:
Once the tart is set, slice into wedges and serve chilled. Optional: Garnish with cinnamon or additional citrus zest.