Preheat your oven to 425°F (220°C).
In a mixing bowl, combine flour, sugar, baking powder, and salt.
Add the sourdough starter and buttermilk to the dry ingredients.
Mix until just combined, being careful not to overmix.
Fold in the melted butter until incorporated.
Turn the dough onto a floured surface and gently knead a few times.
Pat the dough to about 1-inch thickness.
Use a biscuit cutter to cut out biscuits and place them on a baking sheet.
Brush the tops with additional melted butter if desired.
Bake for 15-20 minutes until golden brown.