Rub the flour and butter together until the mixture resembles coarse crumbs. Add the sugar, egg yolk, and a little water just until the dough comes together. Chill for 30 minutes, then bake in a buttered and floured tart pan.

Whisk the egg yolks and sugar together well, then add the cornstarch.

Heat the milk with instant coffee (such as Nescafé) in a saucepan.

In a bowl, mix the egg yolks and sugar, then add the flour last.

Pour half of the hot milk over the egg yolk, sugar, and cornstarch mixture, whisking constantly. Return everything to the saucepan and stir continuously until the mixture thickens. Cool on a tray, then add the cold butter at the end and use to fill your tart.