Mix Dry Ingredients: Whisk together flour, cornstarch, salt in a large mixing bow. Add the leaf lard by the tablespoonful and butter cubes.
Cut in the Fat: Add the leaf lard by the tablespoonful and butter cubes. Cut the fats into the flour using pastry blender or scrapers until coarse crumbs form. The mixture should hold shape when lightly pressed.
Make Egg-Vinegar Mixture: Crack an egg in a glass measuring cup (2 cup, about 480 ml, size), and add vinegar. Pour cold water to measure the liquid to reach 2/3 cup (158 ml) amount. Beat the mixture well.
Add Liquid to Dry Mixture: Drizzle about 1/2 cup (120 ml) of liquid around the crumb mixture and stir gently with a fork. Add more liquid only to dry spots until the dough is moistened (not sticky). Some crumbs are okay.
Gather and Chill Dough: When the dough mixture is moistened without being too sticky or dry, dump it on a lightly floured surface. Gather up the dough. Divide the dough in half. (If I am making a double crust pie, I usually make the bottom crust dough to be slightly larger than the top crust portion) Shape into disks, wrap in plastic, and chill for at least 1 hour.
Roll Out the Dough: If the chilled dough feels too firm, give it a few firm taps with the rolling pin to loosen. Lightly flour your work surface and rolling pin. Roll the dough from the center outward, turning it a quarter turn after each roll to keep it even. Aim for a 13-inch (33 cm) circle. Slide your fingers under the dough occasionally to prevent sticking, dusting with a touch more flour only if needed.
Flute the Edges: Transfer the pastry to fit into a pan, gently nudging it down the sides of the pan. Lift up the overhanging dough and fold it under. Create the flute by using your thumb and forefinger to make a V pattern. Cover with plastic and refrigerate until ready to use.