Preheat the oven to 425 degrees F. Place heavy cast-iron skillet into oven to preheat. Chill a large mixing bowl, pastry cutter, flour and 1 stick butter in the freezer 15 minutes before assembling the biscuits.
Add the flour to the chilled mixing bowl. Working quickly, cut 1 stick butter into the flour using the chilled pastry cutter (or your fingers); the goal here is pea-size balls of butter. In batches, add in the buttermilk until the dough is cohesive yet still moist. Turn the dough out onto a floured surface and lightly press into a 1-inch-thick round. Using a biscuit cutter or business end of a dry-measuring cup, cut out 8 biscuits. (Use the extra dough to cook up a big piece of shortcake for dessert....strawberry shortcake!)
Carefully remove the skillet from the oven and add the oil. Place the dough rounds into the hot skillet, right up against each other, and brush with the 3 tablespoons melted butter. Bake until golden brown, about 15 minutes.