1. Adjust oven rack to upper-middle position and heat oven to 400 degrees. Whisk flours, sugar, baking soda, cream of tartar, and salt in large bowl. Work softened butter into dry ingredients with fork or fingertips until texture resembles coarse crumbs.
2. Add buttermilk and stir with a fork just until dough begins to come together. Turn out onto flour-coated work surface; knead until dough just becomes cohesive and bumpy, 12 to 14 turns. (Do not knead until dough is smooth, or bread will be tough.)
3. Pat dough into a round about 6 inches in diameter and 2 inches high; place on greased or parchment-lined baking sheet or in cast-iron pot, if using. Place the loaf on a cookie sheet and cut a cross shape into the top.
4. Bake until golden brown and a skewer inserted into center of loaf comes out clean or internal temperature reaches 180 degrees, 40 to 45 minutes. Remove from oven and brush with melted butter; cool to room temperature, 30 to 40 minutes.