Instructions

• Preheat the oven to 425°F. Line a baking sheet with parchment paper, or very lightly oil a 10-inch cast iron skillet.
• In a large bowl, whisk together the flour, sugar, baking powder, salt, and baking soda.
• Add the cold butter. Cut or toss it into the flour with your fingers, a pastry cutter, or two forks until the mixture has some pea-sized pieces and some larger flat bits. Do not work it in completely; visible butter pieces are good. (For the softest biscuits, grate frozen butter on the large holes of a box grater, toss it through quickly, and chill the bowl 5 to 10 minutes if it softens.)
• In a separate bowl, stir together the sourdough discard, egg, and sour cream until mostly smooth.
• Pour the discard mixture into the flour mixture. Gently stir with a fork or spatula until the dough starts to clump together.
• Add cold milk 1 tablespoon at a time, just until the dough is soft, shaggy, and holds together when squeezed. It should feel slightly tacky but not wet. A few dry crumbs in the bowl are fine.
• Turn the dough out onto a lightly floured surface. Gently press it into a rough rectangle. Fold it in half or in thirds, then gently pat it out again. Do this only once or twice for soft biscuits.
• Pat the dough to about 1 inch thick. Cut with a sharp biscuit cutter, pressing straight down without twisting, or cut into squares with a sharp knife to avoid scraps.
• Place the biscuits touching or almost touching on the prepared pan so they rise straight with soft sides.
• Bake for 14 to 17 minutes, until the tops are golden and the centers are set. Tap the center biscuit to check that it feels set, not squishy, and that the bottom is lightly golden.
• Brush the warm biscuits with melted butter right after baking. For extra-soft tops, loosely cover with a clean kitchen towel for 5 to 10 minutes. If using cast iron, move the biscuits out of the pan after 2 to 3 minutes.