Sift the flour into a large bowl.
Work the butter into the flour with the tips of your fingers, until the mixture has the appearance of coarse breadcrumbs.
Continue kneading, adding the hot water.
Roll the dough into a large ball, cover with wax paper and refrigerate while preparing the custard filling.
Lightly beat the eggs, taking care not to produce any air bubbles.
Stir in the milk and the sugar.
Add the yellow food color if using.
On a large, lightly floured surface, roll out the dough until it is thin.
Use a pastry cutter to cut out circles.
Fit the circles into the tart shells.
Pour the filling into the shells.
Bake until the custard is cooked and a knife inserted in the middle comes out clean, about 35 minutes.