Grate your butter into a freezer safe bowl. Add in the freezer for at least 30 minutes, or even overnight if more convenient.
Preheat oven to 425°F with the rack in the upper third. Line a baking sheet with parchment.
Whisk the flour, baking powder, baking soda, sea salt, and sugar in a mixing bowl.
In a second bowl, whisk the sourdough discard with the soy milk.
Work the frozen grated butter into the flour mixture with your hands until well distributed without melting.
Add the discard mixture and gently mix until the dough is just combined. Add a splash of milk if dry or a little flour if sticky.
Turn the dough onto a well floured surface. Pat into a 1-inch thick round or rectangle.
Cut biscuits using a cutter or mason jar lid. Transfer to the baking sheet, leaving about 2 inches of space.
Freeze for 15–20 minutes for the flakiest texture.
Bake for 15–18 minutes or until golden on top and cooked through. Let cool before serving.