Preheat oven to 425°F. Line a baking sheet with parchment paper.
In a large bowl, whisk together flour, baking powder, baking soda, salt, and sugar.
Add the cold cubed butter to the flour mixture. Use a pastry cutter, fork, or your fingers to work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Pour in the cold buttermilk and gently stir until the dough just comes together. The dough should be slightly shaggy, not smooth. If it feels too dry, add 1 tablespoon of buttermilk at a time until it holds together.
Turn the dough onto a lightly floured surface. Gently pat it into a rectangle. Fold the dough in half, pat it down, and repeat 3–4 times. This creates flaky layers without overworking the dough.
Pat the dough to about ¾ to 1 inch thick. Use a biscuit cutter or round glass to cut biscuits. Press straight down without twisting so the biscuits rise evenly.
Place biscuits close together on the prepared baking sheet for soft sides, or spaced apart for crispier edges. Bake for 12–15 minutes, or until tall and golden brown.
Brush warm biscuits with melted butter and serve immediately.