Heat the oven to 450°F and line a heavy sheet pan with parchment paper.
Whisk the flour, baking powder, baking soda, salt and sugar in a wide bowl.
Grate the frozen butter through the large holes of a box grater into the flour and toss until every shred is coated.
Pour in the cold buttermilk and stir with a fork until a shaggy dough forms with a little dry flour left at the bottom.
Turn the dough onto a floured counter, pat it into a 1-inch-thick rectangle, fold it in thirds and pat it out once more.
Cut eight biscuits straight down with a 2 1/2-inch cutter, arranging them with their sides touching on the pan.
Bake until tall and well browned on top, 14 to 16 minutes, then brush with the melted butter.
