Step 1

Adjust an oven rack to the upper-middle position and preheat oven to 400 degrees F.

Step 2

In a large bowl, whisk together the flours, sugar, baking soda, cream of tartar, and salt. Work the softened butter into the dry ingredients with a fork or your fingertips until the texture resembles coarse crumbs.

Step 3

Add the buttermilk and stir with a fork just until the dough begins to come together. Turn out onto a floured work surface and knead just until the dough becomes cohesive and bumpy, 12 to 14 turns. (Do not knead until the dough is smooth, or the bread will turn out tough.)

Step 4

Pat the dough into a round about 6 inches in diameter and 2 inches high; place on a parchment-lined baking sheet or in a 12-inch cast iron skillet. Score the dough by cutting a cross shape on the top of the loaf.

Step 5

Bake until the loaf is golden brown and a skewer inserted into the center comes out clean, or the internal temperature reaches 180 degrees F on an instead-read thermometer, 40 to 45 minutes. Remove the loaf from the oven and brush the surface with the melted butter. Cool to room temperature before slicing, about 30 to 40 minutes. Leftovers should be wrapped in plastic wrap and stored at room temperature for up to 3 days.