Preheat your oven to 450°F (232°C) and line a baking sheet with parchment paper. Measure all ingredients before starting, ensuring your buttermilk and butter remain ice-cold until needed.
In a large bowl, whisk together flour, baking powder, salt, baking soda, and sugar (if using) until well combined.
Grate frozen butter directly into dry ingredients using the large holes of a box grater. Toss gently with your fingers until the mixture resembles coarse meal with visible butter flakes.
Create a well in the center of your mixture and pour in the cold buttermilk. Stir gently with a fork until just combined—approximately 15 strokes. The dough should look shaggy and slightly moist.
Turn the dough onto a lightly floured surface and gently press into a rectangle about 1-inch thick. Fold the dough in thirds, like a letter, then rotate 90 degrees and repeat the folding process. Repeat once more for maximum flakiness.
Pat the folded dough to a 1-inch thickness and cut using a sharp-edged 2½-inch round cutter. Press straight down without twisting. Place biscuits on the prepared baking sheet with sides touching for higher rise (or 1 inch apart for crispier sides).
Brush tops with melted butter and bake at 450°F for 12-15 minutes until golden brown. Cool on a wire rack for 5 minutes before serving.