In a mixing bowl, combine the all-purpose flour, egg, melted butter, powdered sugar, and salt. Mix until the dough comes together. Be careful not to overwork the dough.

Wrap the dough in plastic wrap or parchment paper, and refrigerate for at least 30 minutes.

Preheat the oven to 400°F. Meantime, in a mixing bowl, whisk the eggs and powdered sugar together until well combined. Add the milk (not the evaporated milk), cold water, and vanilla extract. Whisk until smooth. Optional: Pass the mixture through a fine-mesh sieve to ensure a smoother custard texture.

Take the dough out of the refrigerator. Shape the dough into many small balls. Gently press each dough circle into the tart molds with your fingers, ensuring an even thickness and covering the bottom and sides. You can also use a small measuring cup to press the dough into the molds and create a well-defined tart shape.

Pour the custard filling into each tart shell, filling them about 80% full. Place the filled tart molds on a baking sheet or tray. Bake in the preheated oven for approximately 30 minutes or until the filling is set and the crust is golden brown. Remove the tarts from the oven and let them cool in the molds for a few minutes before transferring them to a wire rack to cool completely.

Serve and enjoy.