Adjust rack to center position and heat oven to 450 degrees F.

Sift both flours, baking powder, baking soda, sugar and salt together into a mixing bowl or workbowl of food processor fitted with steel blade. Using your fingertips, pastry blender, or steel blade of food processor, rub, cut, or process butter into dry ingredients until mixture resembles coarse crumbs with some slightly larger lumps of butter. (If using your fingers to rub in the butter, don't let your fingertips touch or the friction will melt the butter.) Make a well in the center and pour in 3/4 cup cold buttermilk. Mix with a spatula or fork until mixture forms a soft, slightly sticky ball (adding additional buttermilk as necessary for dough clumps to form ball). Do not overmix. If using a food processor, add buttermilk and pulse process until dough gathers into moist clumps. Remove and form into a ball.

Divide dough into 12 equal portions. Working quickly with lightly floured hands, gently bat a portion of dough back and forth between hands until a rough ball begins to form. Cup palms and lightly pat dough ball to flatten slightly, and form into a rough round. Repeat with remaining dough and place formed dough rounds about 1-inch apart on an ungreased or parchment-lined baking sheet. Carefully brush dough tops with 2 tablespoons melted butter. (At this point, biscuits can be loosely covered with plastic wrap and refrigerated for up to 2 hours.) Bake on the center rack of a preheated 450 degree F oven until biscuits are light golden, about 10 to 12 minutes. If desired, brush the hot biscuits with additional melted butter. Serve immediately.