In a large mixing bowl, combine all-purpose flour and salt. Cut in cold unsalted butter until the mixture resembles coarse breadcrumbs. Stir in granulated sugar, beaten egg, and a splash of cold water. Mix until a dough forms, shape into a disc, wrap in plastic, and refrigerate for 30 minutes.

While the pastry chills, preheat your oven to 400°F (200°C).

In a saucepan, gently heat whole milk until it starts to simmer. In a separate bowl, whisk together egg yolks and granulated sugar until light in color. Gradually pour the hot milk into the egg mixture while whisking continuously to prevent curdling, then transfer to a spouted container.

Grease tart pans with softened butter. Roll out chilled pastry to ⅛ inch thick. Cut circles that fit your tart pans and gently press the pastry into the molds, patching any tears.

Divide custard mixture evenly among pastry shells, filling just below the rim. Sprinkle freshly grated nutmeg over each tart.

Bake at 400°F (200°C) for 10-15 minutes, then reduce temperature to 350°F (176°C) and continue baking for an additional 10 minutes until the custards are set with a slight wobble.

Remove tarts from the oven and cool in the pans for a few minutes before transferring to a wire rack to cool completely.