Set oven to 400°F with rack on lowest level.
Combine flours, sugar, baking soda, cream of tartar, and salt.
Cut in softened butter until crumbly.
Mix until dough forms, adding more buttermilk if needed.
Turn out and form into a round; place in skillet.
Cut a cross on top and bake for 35–40 mins.
Brush with melted butter and cool before slicing.
