1) Adjust oven rack to upper-middle position and heat oven to 400 degrees.

2) Put flours, sugar, baking soda, cream of tartar, and salt in food processor. Pulse 4-5 times to combine and aerate.

3) Add butter and pulse until until texture resembles coarse crumbs (no large pieces of butter should remain).

4) Pour food processor ingredients into a bowl, add buttermilk and stir with a fork just until dough begins to come together.

5) 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.)

6) Pat dough into a round about 4 inches in diameter and 1 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.

7) Bake until golden brown and a skewer inserted into center of loaf comes out clean or internal temperature reaches 180 degrees, 20 to 25 minutes.

8) Remove from oven and brush with melted butter; cool to room temperature at least 10 minutes before attempting to cut.