Preparations: Preheat the oven to 325 degrees Fahrenheit and grease the tart pans.
Cut the lard. Use a sifter to sift the flour into a large bowl. Work the lard and 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 very thin (about 1/8-inch or 3 to 4 mm thick). Cut the dough in half before rolling out if needed. Use a pastry cutter to cut out 18 circles that are 3-inches in diameter. 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 minute). Cool