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Preheat the oven to 450°F. If you have a 9” or larger cast iron skillet, rub it with a bit of vegetable shortening or oil and place it in the top third of your oven. Otherwise, lightly grease a baking sheet, or line it with parchment and set it aside; there’s no need to preheat it.
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To make the biscuit dough: For best results, weigh your flours; this recipe was developed by metric weight. However, if you prefer to work with volume measurements, please be sure to measure your flour the King Arthur way: gently spoon the flour(s) into a cup, then sweep off any excess.
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To mix by hand or with a mixer: In a medium bowl, whisk together the flour, baking powder, and salt. Work in the cold butter, mixing until fairly well combined but with some small chunks of butter remaining intact.
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Stir in the starter, then the milk or cream, adding enough to make a cohesive, firm dough.
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To make the biscuit dough using a food processor: Using the dough blade, pulse together the flour, baking powder, and salt. Add the cold butter and pulse briefly and repeatedly until fairly well combined but with some small chunks of butter remaining; this happens pretty quickly, so watch carefully. Add the starter and milk or cream. Using short bursts, pulse repeatedly just until the dough starts to come together.
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To shape the biscuits: Transfer the dough to a greased piece of parchment or waxed paper, or other greased work surface.
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Knead it gently several times to smooth it out. Fold it in thirds like a letter, then pat it into a rough 4” x 8” rectangle; it should be a scant 3/4" thick.
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Flip the rectangle over so the smoother side is on top. Cut eight square biscuits, or use a 2” biscuit cutter to cut rounds, patting together the scraps as necessary to cut additional biscuits.
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Wrap the biscuits in the parchment or waxed paper and place them in the freezer while the oven finishes preheating.
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To bake the biscuits: Remove the hot skillet from the oven. Unwrap the biscuits and arrange them in the skillet (or on the baking sheet). Place the skillet or pan on the oven’s upper rack.
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Bake the biscuits for about 12 to 14 minutes (if you’ve used regular dairy products), or 15 to 17 minutes if you’re using plant-based milk and butter. The biscuits should be lightly browned.
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Remove the biscuits from the oven and brush them with melted butter, if desired; brown butter is especially yummy.
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Storage information: Allow the biscuits to cool completely, then store airtight at room temperature for several days. Freeze, well-wrapped, for longer storage.
