1. Preheat a convection oven to 350 degrees. In the bowl of a very large electric mixer fitted with the dough-hook attachment, combine buttermilk, shortening, sugar, and baking powder. Beat or stir to combine. Add flour, a cup at a time, beating just to combine after each addition.

2. Lightly coat two rimmed baking sheets with cooking spray. Divide dough in two. On a lightly floured surface, roll out one half of dough to about 3/4 inch thick. Use a lightly floured biscuit cutter to cut dough into 2 1/4-inch rounds. Transfer to prepared baking sheet. Brush tops lightly with melted butter. Repeat with second half of dough. Bake until light-golden brown and cooked through, 12 to 16 minutes.