Pre-heat the oven to 425ºF.

Combine the dry ingredients in a bowl and whisk to break up any lumps.

Cut the butter into small cubes and pinch it into the dry ingredients with your fingers until the butter cubes break down into crumbs.

Combine the sourdough starter and buttermilk in a bowl, stirring well to combine.

Add the buttermilk and starter mixture to the dry ingredients and stir to combine, just until there are no more streaks of dry flour.

Turn the mixture out onto a floured surface and press it into a rectangle that is about ¾-inch thick. Fold the dough like you are folding a letter, folding the top of the dough down and the bottom of the dough up. Give the dough a 90º turn and pat it down again into a rectangle about ¾-inch thick. Fold again. Fold the dough over on itself three or four times and press it down one last time.

Use a round cutter to cut the dough into circles and place them on a baking sheet. Transfer the baking sheet to the oven and bake for 15 minutes, until they have risen and browned on top.

Brush the tops of the biscuits with the melted butter and serve warm or cooled to room temperature.