Preheat oven to 400°F and position rack to lowest level.
In a large bowl, whisk together flours, sugar, baking soda, cream of tartar, and salt.
Cut softened butter into dry ingredients using fingers or a pastry blender until mixture resembles coarse crumbs.
Pour in the buttermilk and stir just until no dry flour remains. Add 1–2 tablespoons extra buttermilk if dough seems dry.
Turn dough onto lightly floured surface and gently shape into a 6-inch round. Do not overwork.
Transfer dough to cast iron skillet or parchment-lined baking sheet. Score a deep “X” across the top with a serrated knife.
Bake 35–40 minutes or until loaf is deep golden and sounds hollow when tapped on the bottom.
Remove from oven and brush top with melted butter. Cool on rack for 30–45 minutes before slicing.
