• Line a baking pan with parchment paper (if needed), and set aside.

Make Vegan Buttermilk:

• Pour the dairy-free milk into a measuring cup or bowl.
• Add the apple cider vinegar to the milk and stir.
• Place in the refrigerator to keep cold, for at least 10 minutes, while you prepare the dry ingredients.

Make Biscuit Dough (work quickly to keep the dough cold and the butter from melting):

• In a medium bowl, whisk together the flour, baking powder, baking soda, sugar, and salt.
• Add the frozen grated butter to the mixture and stir. Break up any large pieces of butter that have stuck together with the mixing spoon.
• Slowly pour the rested, chilled buttermilk into the dry ingredients and stir just until a rough, shaggy dough forms. A few dry spots are fine. Avoid overmixing.
• Transfer the dough to a floured pastry mat or parchment paper.
• Flour your hands and press the dough together (don't knead it). If the dough is too sticky, sprinkle more flour on top. Pat it into a rectangle about ¾ of an inch thick. The shorter edge should be facing you.
• Fold the dough into thirds, like you would fold a letter, by bringing the top third down to the center, then fold the bottom third up over the top. If there are any loose bits of dough, place them on top before continuing.
• Flour the pastry mat again. Turn the dough 90 degrees, shape it into a rectangle again, and repeat the folding process.
• Repeat the patting, folding, and turning two more times for a total of four rounds.
• The dough should look smoother now. Gently pat it into an even rectangle about ¾ inch thick and roughly 6 inches wide by 9 inches long.
• Dip a 2.5-inch round cutter in flour, then cut out one biscuit. Press straight down and lift straight up without twisting or wiggling the cutter. Dip the cutter in flour again before cutting the next one, and repeat until you have 6 biscuits. Place the biscuits on a plate after you have cut them.
• Gather the remaining scraps and press them together, it should be enough for three more biscuits. Fold the dough in half and pat it into a ¾ inch thick square, with a surface big enough to cut one biscuit. Flour the cutter and cut the biscuit, placing it on the plate. Repeat two more times.
• Place the plate with the biscuits in the refrigerator to chill for 30 minutes.
• Preheat oven to 425°F (220°C).

Bake:

• After chilling, remove the plate from the fridge and place the biscuits in a corner of the baking pan so the sides can support them as they rise. Arrange them close together, almost touching.
• Brush the tops with dairy-free milk.
• Bake for 17-18 minutes, until golden brown. Don't overbake.
• Remove the pan from the oven and let the biscuits cool for a few minutes. Then transfer them to a cooling rack and serve warm.