Preheat the oven to 425°F, making sure your rack is in the upper third. Grease a baking sheet (you can also line it with parchment paper or silpat) or cast iron skillet. 

Combine dry ingredients - flour, baking powder, and salt in a large mixing bowl, then work the butter into the flour until the mixture is unevenly crumbly.

Add the starter and mix the dough just until the ingredients come together. Some lumps of butter in the dough are a good sign – they will melt during baking and create air pockets. Over mixing the dough can make biscuits tough to avoid over mixing!

Turn the dough out onto a lightly floured surface and gently pat it into a circle about 1"-thick.

Use a sharp 2 3/8" biscuit cutter to cut rounds as close to one another as possible. Gently push and pat the scraps into a thick rectangle. Cut out more biscuits. If you have any leftover scraps, shape them into a 1"-thick biscuit as well.

Place the biscuits on the greased baking sheet or cast iron skillet. Give them lots of space so they spread as they bake. 

Bake the biscuits in the upper third of your oven for 20 to 23 minutes, until they're golden brown.

Remove the biscuits from the oven, and serve warm. Top with melted butter for the best homemade sourdough biscuits!