To make the pretzel coating: In a small bowl, combine the hot water and baking soda, stirring to dissolve most of the baking soda. Set aside. , To make the biscuit dough: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. , In a large bowl, combine the flour, baking powder, sugar, and salt.  , Using your fingertips, a fork, a pastry blender, a stand mixer, or a food processor, work the butter into the flour mixture; your goal is an evenly crumbly mixture (think breadcrumbs), but some larger, pea-sized chunks are OK. , Drizzle 1 1/2 cups (340g) of the buttermilk evenly over the flour mixture. Using a flexible spatula or a bowl scraper, quickly and gently mix for about 15 seconds, until you've made a cohesive dough. If the mixture seems dry and won't come together, add up to 2 tablespoons (28g) buttermilk to make it cohesive. Some dry bits of dough are OK. , Place the dough on a lightly floured piece of parchment. Pat it into a rough rectangle about 3/4" thick, or a 10" x 8" rectangle. Fold one-third into the center, then the opposite third over the first, like you were folding a business letter. Pat or roll the dough gently with a floured rolling pin until it’s 3/4" thick again., Fold the dough once more into thirds and pat or roll it out to a rectangle about 3/4" thick, about 10" x 8" in size. Use the parchment to transfer the dough to a baking sheet., Give the coating mixture a final stir, then brush it liberally onto the entire top surface of the dough. , Transfer the baking sheet to the freezer and allow dough to chill for 30 minutes, uncovered. (See “tips,” below for details on how to prepare biscuits the night before and bake the next morning.) While the dough chills, preheat the oven to 425°F with a rack in the upper third.  , To top and bake the biscuits: Once chilled, brush the egg wash evenly onto the surface of the dough and sprinkle with salt. , Use a bench knife or chef’s knife to slice the dough into a 3 x 4 pattern to create a dozen 2 1/2" x 2 1/2" squares. Separate the squares with about 1 inch between them. , Bake the biscuits for 15 to 20 minutes, or until the tops are dark brown. Remove them from the oven and serve warm. , Storage information: Store any leftover pretzel biscuits, well wrapped, at room temperature for several days. Freeze for longer storage. Biscuits are always best when they're rewarmed before serving.