Step 1

Preparation Preheat oven to 425°. Stir together baking powder, baking soda, kosher salt, sugar, and 1½ cups flour with a fork in a large bowl. Add ½ cup chilled butter and toss with your hands or fork just to coat. Using your fingers, smash butter into flat disks (if you miss a few, it will be fine). Using a pastry cutter or fork, work butter into dry ingredients until shaggy crumbles form (you should have some large pieces, some small pieces, some flat pieces, and some sandy flour).

Step 2

Add sourdough starter and mix gently with fork to incorporate, then fold with your hands just until dough comes together with just a few crumbly pieces in the bottom of the bowl.

Step 3

Transfer dough to a well-floured surface and pat out with your hands until about ½” thick (the shape doesn’t matter too much at this point). Fold into thirds as you would a letter to create a rough rectangle. Working from short sides, fold in thirds like a letter again. Pat dough out to a ½”-thick square. Repeat folding process. Pat out dough for a third time to a 1”-thick square—it should feel airy, like a pillow at this point. Using a floured 2½”-diameter biscuit cutter or glass, punch out as many biscuits as you can (do not twist cutter). Transfer biscuits to a parchment-lined rimmed baking sheet, spacing at least 1” apart.

Step 4

Using your hands, gently press scraps into a rectangle, then fold in half. Pat out to a 1”-thick rectangle and cut out more biscuits. Gather remaining scraps together to form 1 final biscuit (you should have 10 total). Transfer to baking sheet. Brush biscuit tops with 2 Tbsp. melted butter and sprinkle with sea salt if desired

Step 5

Bake biscuits until tall and golden and an instant-read thermometer inserted into the side of a biscuit registers 205°F), 12–15 minutes.