Heat oven to 400 degrees.

Whisk all purpose flour, cake flour, sugar, baking soda, cream of tartar, and salt together in large bowl. Add softened butter and use your fingers incorporate into flour mixture. Make a well in center of flour and pour in 1 1/2 cups buttermilk. Stir with fork until completely combined. Add buttermilk one tablespoon at a time until all flour is moistened.

Place dough on a lightly floured surface and shape into a rough 6 or 7 inch round. Dough will look uneven and shaggy.

Place dough in a 12 inch cast iron skillet. Score a cross, about 5 inches by 5 inches and 3/4 inch deep. Bake until lightly browned and knife inserted in the oven comes out clean, about 40 minutes. Brush with melted butter, if desired.

Let cool for at least 30 minutes before serving.