Step 1: Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper or lightly grease it. If possible, chill your mixing bowl and flour for 10-15 minutes.
Step 2: In a large bowl, whisk together the flour, baking powder, baking soda, sugar (if using), and salt until well combined.
Step 3: Add the cold, cubed butter to the flour mixture. Using a pastry blender, your fingertips, or a food processor, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. This step is crucial for flaky biscuits!
Step 4: Pour the cold buttermilk into the flour mixture. Stir with a wooden spoon or a spatula just until the dough comes together. Do not overmix; the dough should be shaggy and slightly sticky.
Step 5: Turn the dough out onto a lightly floured surface. Gently pat the dough into a rough rectangle, about 1 inch thick. Perform the 'fold and turn' technique: fold one-third of the dough over the center, then fold the remaining third over that. Rotate the dough 90 degrees and repeat this folding process one or two more times. This builds layers without overworking the dough.
Step 6: Gently pat or roll the dough to a thickness of about 3/4 to 1 inch. Using a 2.5-inch biscuit cutter, press straight down firmly through the dough. Do not twist the cutter, as this seals the edges and prevents proper rising. Rework any dough scraps by gently pressing them together and cutting out additional biscuits.
Step 7: Place the cut biscuits onto the prepared baking sheet, ensuring they are touching slightly for softer sides, or spaced apart for crispier sides.
Step 8: Bake for 15-18 minutes, or until the biscuits are golden brown on top and cooked through. If desired, brush the tops with melted butter immediately after removing them from the oven.
Step 9: Serve warm and enjoy your perfectly flaky, buttery buttermilk biscuits!