Preheat oven to 500 degrees F. Generously spray a 9-inch round cake pan and a 1/4-cup dry measure with nonstick cooking spray and set aside. In a food processor, combine 2 cups of flour, baking powder, sugar, salt and baking soda and pulse to incorporate well. Add butter cubes to flour mixture and pulse until mixture resembles pebbly, coarse cornmeal. Transfer mixture to a bowl then add buttermilk and stir mixture with rubber spatula until just incorporated. Dust a baking sheet with the remaining flour. Using a well greased 1/4-cup dry measure, scoop level amount of dough onto floured baking sheet. Repeat with the remaining dough, forming 12 evenly sized mounds. Lightly dust the top of each piece of dough with flour. With floured hands, gently pick up each piece of dough, coating the outside with flour. Shape each piece of dough into a round ball, shake off excess flour and place into the prepared cake pan. Repeat with remaining dough, arranging 9 rounds of dough around the perimeter of the cake pan and 3 in the center. Brush rounds with hot melted butter, taking care not to flatten them. Bake 5 minutes then reduce oven temperature to 450 degrees F. Continue to bake 15 minutes or until biscuits are deep golden brown. Transfer biscuits to a rack and let cool 5 minutes before serving.