Preheat oven to 450F. Place butter in a 9x9-inch baking dish and melt it in the oven while preheating.
Whisk together the flour, baking powder, sugar, and salt in a large bowl.
Pour in the cold buttermilk and stir just until combined into a thick, wet batter.
Remove the hot dish and pour the batter directly into the melted butter without stirring it in.
Score the batter into 9 squares without cutting through to the pan.
Bake for 25-28 minutes until deeply golden-brown with crisp edges.
Let sit in the pan for 5 minutes, then cut along the scored lines and lift out.