Meanwhile, spray a 12-cup muffin tin with cooking spray. In a food processor, pulse the flour with 2 tablespoons of the sugar, the baking powder, baking soda and salt. Add the cubed butter and pulse until it is the size of small peas. Add the buttermilk and pulse a few times, just until a soft dough forms. Turn the dough out onto a floured surface and knead 3 times. Pat or roll the dough into an 8-by-12-inch rectangle. Brush with 2 tablespoons of the melted butter.