Preheat oven to 450°F (preheat cast iron skillet too, if using).
Whisk flour, baking powder, baking soda, salt, and sugar in a large bowl.
Grate frozen butter into the flour; toss to coat shreds.
Stir in cold buttermilk just until a shaggy dough forms. Do not overmix.
On a floured surface, pat into a rectangle; fold in thirds like a letter, turn 90°, and pat out. Repeat 3-4 times.
Pat to 3/4-inch thick; cut straight down with a floured cutter (no twisting). Press scraps together for extra biscuits.
Arrange touching in the hot skillet; brush tops with buttermilk. Bake 13-15 minutes until golden. Brush with melted butter and serve warm.