In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
Pour in the buttermilk and stir just until the dry ingredients are moistened. Don't overmix.
Pour the melted butter into an 8x8-inch baking dish.
Spoon the biscuit dough over the melted butter and gently spread it into an even layer over the top. The butter will rise around the edges, and that's ok.
Using a bench scraper or sharp knife, cut the dough into 9 equal squares. I usually go over the cuts a second time to make the score marks a little more visible. The dough will bake back together, but the scoring makes the biscuits much easier to separate after baking.
Bake for 22 to 25 minutes, or until the top is golden brown.
Let the biscuits rest for several minutes before cutting along the score marks and serving warm.
Notes
This recipe works in either an 8x8-inch or 9x9-inch baking dish. I usually reach for my 8x8 because I love the extra-thick biscuits.