Crust (10 min plus 30 min): Rub the butter into the flour, sugar and salt until it looks like breadcrumbs. Mix in the yolk and water until it just comes together. Flatten, wrap and chill 30 minutes.
Line (15 min): Roll to 4 mm and cut 9 cm (3 1/2 in) rounds. Press each into a fluted tart tin or muffin cup, pushing the dough up so it stands slightly above the rim. Chill 15 minutes.
Custard (10 min): Dissolve the sugar in the hot water and cool to lukewarm. Whisk the eggs gently (no froth), then whisk in the sugar water, evaporated milk and vanilla. Strain twice through a fine sieve. Skim off any bubbles; that's what keeps the top glassy.
Bake (25 min): Heat the oven to 200C (400F). Fill the cases almost to the top. Bake 10 minutes, then lower to 160C (325F) and bake 12 to 15 minutes more, until the custard is just set with a slight wobble. If it starts to puff, crack the oven door for a minute.
Cool (10 min): Cool 10 minutes in the tins, then lift out. Best warm, the day they're made.
Tips
Low heat for the custard keeps it smooth; puffing means it's too hot.
For a flaky Bakehouse-style crust, line the tins with puff pastry instead.