Preheat oven to 425°F (220°C); line a sheet with parchment.
Whisk flour, baking powder, baking soda, sugar, and salt.
Cut in cold butter until coarse and crumbly with pea-sized pieces.
Stir in cold buttermilk just until a shaggy dough forms.
Turn out, pat into a rectangle, and fold in thirds 3 times.
Pat to 1 inch thick; cut straight down (no twisting).
Place on sheet, sides touching; chill 10 minutes.
Brush tops with buttermilk or melted butter.
Bake 15–18 minutes until deep golden. Cool 5 minutes and serve.
