Grease or spray a 9-inch tart or pie pan and set it aside. Clear a little freezer space now so the crust can chill without being bumped.
Combine the flour, confectioners' sugar, and salt for the crust. Add the cold butter and work it in until the mixture looks sandy in places and starts to hold together when pressed.
Add the egg yolk and mix only until the dough gathers. Handle it gently; too much working makes the crust tougher than Ouma wants it.
Press the dough evenly into the bottom and sides of the pan. Try to keep the thickness even, especially at the corners, then freeze the crust for at least 30 minutes.
Heat the oven to 205°C / 400°F. Bake the chilled crust for 20–25 minutes, until it looks dry and golden. Set the shell aside while you make the filling.
Warm the milk, butter, and nutmeg or additional cinnamon in a saucepan until hot and steaming, then remove it from the heat. Do not let it boil hard.
In a separate bowl, whisk the flour, cornstarch, sugar, vanilla, optional almond extract, and eggs until smooth. Scrape the bowl once so no dry pockets hide at the bottom.
Whisk a little of the warm milk into the egg mixture first, then whisk that loosened mixture back into the saucepan. This gentle step helps keep the custard smooth.
Return the saucepan to the stove over moderate heat. Stir constantly, reaching the corners of the pan, until the custard bubbles and thickens; then cook for about 5–6 minutes more while stirring.
Pour the hot custard into the baked pastry shell and smooth the top. If you see a small lump, do not panic — smooth what you can and keep going gently.
Sprinkle the top with cinnamon while the custard is still soft so it settles in nicely. Chill until the tart is cold and firm enough to slice cleanly.
Serve chilled in neat slices. Ouma would rather wait a little longer than cut it warm and watch the custard run.