Preheat the oven to 425°F. Lightly grease an 8x8-inch baking pan or line it with parchment paper.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, granulated sugar, and kosher salt until evenly combined.
Using the large holes of a box grater, grate the frozen butter directly into the flour mixture.
Gently toss the grated butter with the flour using your fingertips until evenly distributed. Some small pieces of butter should remain visible throughout the mixture.
Pour in the cold buttermilk and gently fold with a spatula just until a shaggy, slightly sticky dough forms. Avoid overmixing.
Transfer the dough to a lightly floured work surface. Gently press it together and shape it into a square approximately 6x6 inches and 1 inch thick. Do not knead the dough.
Using a sharp knife or bench scraper, cut the dough into nine equal squares.
Place the biscuits in the prepared 8x8-inch baking pan in three rows of three, keeping the biscuits close together.
Bake. Bake for 16–20 minutes, or until the biscuits have risen, the tops are lightly golden brown, and the centers are fully baked.
Remove the biscuits from the oven and immediately brush the tops with melted butter.
Sprinkle the warm biscuits with flaky sea salt.
Serve warm with butter, honey, jam, or alongside your favorite breakfast or dinner.