Preheat oven to 400º.

In a large mixing bowl, whisk together the flours, sugar, soda, cream of tartar and salt.

Add soft butter to the flour mixture and rub it into the flour with your fingers until it is incorporated.

Start by adding 1 ½ cups of the buttermilk to the flour mixture. Use a fork to combine until there is no more flour at the bottom of the bowl. Add additional buttermilk if needed.

Place on a lightly floured surface and pat it into a 6 inch round.

Place dough in a 12 inch cast-iron skillet. Cut a cross on the top, 5 inches long and ¾ inch deep.

Bake until golden brown and a knife inserted in the center comes out clean, about 40-45 minutes. Remove from oven and immediately brush with a tablespoon of butter. Let cool for 30 minutes before slices.