Preheat oven to 450°F (232°C).
In a medium bowl, whisk together flour, sugar, baking powder, and salt.
Pour in the buttermilk and stir until just combined. Do not overmix.
Pour the melted butter into an 8x8-inch baking dish.
Spoon the biscuit dough directly on top of the melted butter and gently spread it to the edges.
Using a sharp knife, cut the unbaked dough into 9 squares to help with portioning later.
Bake for 20–25 minutes or until golden brown on top.
Let cool slightly before serving warm.