Yields 1 loaf
3 cups all-purpose flour
Adjust oven rack to upper-middle position and preheat oven to 400 degrees F. Grease or parchment-line a baking sheet (a cast-iron skillet can also be used).
In a large mixing bowl, whisk together the flours, sugar, baking soda, cream of taartar and salt. Work in softened butter with fork or fingertips until texture resembles coarse crumbs. Add buttermilk and stir with a fork just until dough begins to come together.
Turn dough out onto floured work surface, knead until dough just becomes cohesive and bumpy, about 12-14 turns. (Do not knead untiol dough is smooth or bread will be tough.) Pat dough into a round about 6 inches in diameter and 2 inches high. Place dough onto prepared baking sheet, or into skillet if using. Using a sharp knife, cut a cross into the top of the dough.
Bake until golden brown and a pick inserted into the center comes out clean or internal temperature reaches 180 degrees F., 40-45 minutes. Remove from oven and brush with melted butter. Cool to room temperaature, about 30 minutes.