Heat the oven. Set a rack in the upper-middle position and heat the oven to 450°F (230°C). Line a rimmed baking sheet with parchment paper.
Mix the dry ingredients. In a large bowl, whisk the flour, baking powder, sugar and salt until evenly combined.
Cut in the butter. Add the cold butter cubes and work them in with a pastry blender or your fingertips for about 2 minutes, until the mix looks like coarse crumbs with plenty of pea-sized butter pieces left. If the butter starts to feel soft or greasy, put the bowl in the fridge for 10 minutes.
Add the buttermilk. Make a well, pour in ¾ cup cold buttermilk and stir with a fork just until a shaggy dough forms and no dry flour is left at the bottom of the bowl. If dry patches remain, add buttermilk 1 tablespoon at a time. The dough should look rough, not smooth.
Fold for layers. Turn the dough onto a lightly floured counter and pat it into a rectangle about 6 x 8 inches and ¾ inch thick. Fold it in thirds like a letter, turn it 90 degrees and pat it out again. Repeat for 3 folds in total; the dough will turn more cohesive and show streaks of butter.
Cut the biscuits. Pat the dough to an even ¾-inch thickness. Dip a 2½-inch biscuit cutter in flour and press straight down without twisting. Gently press the scraps together once and cut again (these rise a little less).
Arrange and chill. Place the biscuits on the sheet with sides just touching for taller, softer sides, or 1 inch apart for crisper edges. Refrigerate for 15 minutes, until the butter feels firm again.
Bake. Brush the tops with buttermilk or melted butter. Bake for 12-15 minutes, turning the pan halfway, until the biscuits are tall and deep golden on top, the bottoms are browned and the layers on the sides look dry rather than doughy.
Rest and serve. Let them cool on the pan for 5 minutes, then serve warm.