If using milk, combine the cold milk with 1 tablespoon white vinegar to make 1 cup. Set aside while you prepare the rest. Alternatively, use 1 cup cold buttermilk.

In a medium mixing bowl, sift together sweet rice flour, almond flour, millet flour, tapioca flour, baking powder, baking soda, and salt. Whisk the flours well to combine. For sweet biscuits, add 1½ tablespoons sugar.

Grate the frozen butter directly into the dry ingredients using a coarse grater. Working quickly, use your hands to toss the butter into the dry ingredients until just evenly distributed. Chill the flour and butter for 10 minutes. Line a large baking
sheet with parchment paper.

Once chilled, create a well in the center of the flour and pour the milk or buttermilk into it. Use a fork to mix gently until evenly distributed, forming a shaggy dough.

Dust a work surface generously with tapioca flour and use a rubber spatula to dump the dough onto the surface. Use tapioca-flour-dusted hands to quickly form it into a ball - it will seem very moist. Dust the top with just a bit more tapioca flour. Working quickly, press into a ½-inch tall rectangle. Use a multipurpose scraper or chopper to create two creases to fold it in thirds like a letter, then again in half. Press out again and repeat the folding three more times, lifting the dough with the scraper and dusting the surface with tapioca flour as you go. Press into 1¼-inch tall round or rectangle. Use a 2-inch round pastry cutter or cookie cutter to cut as many rounds as possible (press down and up, but do not twist!). Using the scraper to lift it from the surface, transfer each biscuit to the lined baking sheet, at least 1½ inches apart. Reroll scraps once, repeating the folding a couple of times until you've cut out 9-10 biscuits. Place the baking sheet in the freezer for 10 minutes or fridge for 30 minutes.