Step 1: Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper or a silicone mat. Alternatively, use a well-greased cast-iron skillet for crispier bottoms.
Step 2: In a large bowl, whisk together the flour, baking powder (if using all-purpose), baking soda (if using all-purpose), and salt. Ensure all dry ingredients are well combined.
Step 3: Add the very cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut or rub the butter into the flour until the mixture resembles coarse meal with some pea-sized pieces of butter remaining. Work quickly to keep the butter as cold as possible.
Step 4: Make a well in the center of the flour-butter mixture. Pour in the cold buttermilk. Stir gently with a fork or rubber spatula until just combined and the dough is shaggy and sticky. Avoid overmixing.
Step 5: Lightly flour a clean work surface. Turn the shaggy dough out onto the surface. Gently pat the dough into a rough rectangle, about 1-inch thick. Fold one third of the dough over the middle, then fold the other third over that, like folding a letter. Rotate the dough 90 degrees, and gently pat it out again to about 1-inch thick. Repeat this folding process 2-3 more times for extra flakiness.
Step 6: Pat the dough out to a final thickness of 1/2 to 3/4 inch. Using a 2-inch or 2.5-inch round biscuit cutter, press straight down through the dough without twisting. Place the cut biscuits onto your prepared baking sheet, either slightly touching for softer sides or spaced apart for crispier edges. Gently re-roll any scraps and cut additional biscuits.
Step 7: For a golden-brown top, brush the tops of the biscuits with a little melted butter or extra buttermilk (optional, but recommended).
Step 8: Bake in the preheated oven for 12-18 minutes, or until the biscuits are beautifully golden brown on top and bottom. Keep an eye on them as oven temperatures vary.
Step 9: Remove from the oven and, if desired, brush with a little more melted butter. Serve warm, straight from the oven.