Cream butter with lard. Add egg and sugar. Beat well. Add flour, 1 cup at a time. The dough will be mealy. Work quickly with your hands to gather dough into a ball. Knead lightly so the mixture adheres. You may chill it at this point while making the filling.
TO MAKE FILLING
Be sure all ingredients are at room temperature. Beat whole eggs with egg yolks well. Beat at low speed. (Do not over beat.) Add sugar, the milk, half and half. Let mixture rest for 10-15 minutes Skim off all the foam from mixture.
ASSEMBLING
Separate dough into 24 balls. Press each into a 2 1/2 inch (measured across top) tart shell to an even layer across the bottom and all the way up the side. Fill tart shell with filling almost to the top.
BAKING
Preheat oven at 300 degrees. Place tarts on cookie sheet and bake for 45 minutes. Cool for 10-15 minutes. Loosen slightly by inserting a toothpick along the sides. The tart shell will unmold easily.