Preheat the oven to 450°F. Sift flour, baking powder, sugar, salt, and cream of tartar together into a large bowl.
Cut in cold butter until the mixture is the consistency of coarse oatmeal. Make a well in the center of the mixture and pour in buttermilk. Stir with a wooden spoon until dough begins to come together.
Bake for 10 minutes, or until golden brown. Brush with melted butter as soon as biscuits come out of the oven. Serve warm.