Preheat oven to 425°F (220°C). Line a rimmed baking sheet with parchment paper.

Freeze butter for about 10 minutes.

In a large bowl, stir together flours, sugar, baking powder, salt, and baking soda. Add cold butter, tossing to coat. Using a pastry blender or 2 forks, cut in cold butter until the mixture is crumbly and the largest butter pieces are pea-size. Using a fork or your fingers, stir in cold buttermilk until a dough forms. (It is OK if there are some dry flour bits remaining.)

Turn out dough onto a clean surface. Pat or roll dough into an 8-inch square (about ¾ inch thick). Roll out and make a letter fold. Repeat 3 more times, finishing with an 8 inch square. Using a chef’s knife coated in flour, trim edges, if desired, to create a 7½-inch square, and cut dough into 2½-inch squares. Place 2 inches apart on prepared pan. Freeze until cold, about 10 minutes.

Brush tops with melted butter.

Bake until golden brown, 15 to 18 minutes. Brush with melted butter, and serve warm.