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1
Sift flour into a medium bowl, then add sugar, instant dry yeast, salt, and baking soda; whisk until well combined (this may take up to 1 minute). Add the butter, toss to break up the pieces, and, using your hands, smash each cube flat. Continue smashing and rubbing until the butter has mostly disappeared into a floury mix, although a few larger, Cheerio-sized pieces may remain. This can also be done with 4 or 5 pulses in a food processor, just take care not to overdo it. The prepared mix can be refrigerated up to 3 weeks in an airtight container, then used as directed below.
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2
Add milk, and stir with a flexible spatula until the flour has been fully absorbed. When the dough forms a rough ball, cover with plastic and set aside at cool room temperature until roughly doubled in bulk, about 2 hours. At temperatures significantly above or below 70u00b0F (21u00b0C), this process will proceed at a different rate, so use the visual cues rather than a specific timetable.
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3
After proofing, turn dough onto a lightly floured surface. With lightly floured bare hands, gently pat the dough into a squarish shape about 1/2 inch thick, then fold in half; repeat twice more for a total of 3 folds, using only enough flour to keep your hands from sticking. Finish by patting the dough to a thickness of 3/4 inch. If needed, dust away any excess flour, then cut into 1 3/4-inch rounds and arrange in a 10-inch cast iron skillet. Gather scraps into a ball, pat and fold a single time, then cut as many more biscuits as you can. The final round of scraps can be gathered and shaped into a single biscuit by hand. Cover tightly with foil and refrigerate overnight, between 8 and 12 hours.
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4
Adjust oven rack to lower-middle position and preheat to 400u00b0F (200u00b0C). Brush the cold dough with melted butter, and bake until the biscuits are well-risen and golden brown, about 20 minutes. Let the biscuits cool about 5 minutes to help set their crumb, then serve as desired, whether alongside soups and stews or split for shortcake or breakfast sandwiches. Leftovers can be stored up to a week in an airtight container. To serve, split the stale biscuits in half, brush with melted butter, arrange on a baking sheet, and broil until golden brown, then serve with jam.
