Preheat oven to 425°F (220°C). Lightly grease a baking sheet or skillet.
Mix dry ingredients: In a bowl, whisk flours, baking soda, and salt.
Add wet ingredients: Make a well in the center, pour in buttermilk (and melted butter if using). Stir until just combined (do not overmix).
Shape dough: Turn onto a floured surface, gently knead into a round loaf (~1.5" thick).
Score: Place on the baking sheet. Cut a deep "X" on top with a knife (helps even baking).
Bake: 35–40 minutes until golden and hollow-sounding when tapped. Cool on a wire rack before slicing.