Preheat the oven to 450°F / 230°C and line a baking sheet with parchment paper.
Combine the self-rising flour and sugar in a large bowl, then cut in the cold butter cubes using a pastry cutter or fork until the mixture resembles coarse crumbs with pea-sized butter pieces.
Pour in the cold buttermilk and stir gently with a fork just until the dough comes together, being careful not to overmix.
Turn the dough onto a lightly floured surface, pat gently to 3/4-inch thickness, and cut out biscuits using a 2.5-inch round cutter, pressing straight down without twisting.
Place the biscuits on the prepared baking sheet with sides touching and bake for 12 to 15 minutes until the tops are golden brown.
Brush the hot biscuits with melted butter immediately after removing from the oven and serve warm.