Stir the apple cider vinegar into 1 cup of the unsweetened soy milk and set it aside for 10 minutes, until the milk looks slightly thickened and curdled at the surface.

Whisk the flour, baking powder, baking soda, sugar, and salt together in a large bowl. Put the bowl in the freezer for 10 minutes along with the stick of vegan butter if you have room.

Take the frozen vegan butter and grate it on the large holes of a box grater directly into the flour. Toss the shreds with your fingers until every piece is coated and the mixture looks like coarse crumbs with a few pea-size lumps left in it.

Pour in the soy buttermilk and fold with a spatula just until no dry flour is left at the bottom of the bowl. The dough should look shaggy and slightly tacky, not smooth.

Turn the dough onto a lightly floured counter and pat it into a rectangle about 1 inch thick. Fold the rectangle in thirds like a letter, pat it out again, and repeat the fold once more.

Pat the dough to 1 inch thick and cut it straight down into 8 squares with a sharp knife, pressing down without dragging the blade. Chill the squares for 15 minutes if the dough feels warm at all.

Heat the oven to 425 degrees F and line a baking sheet with parchment. Set the squares on the sheet so they touch each other, brush the tops with the remaining soy milk, and bake for 15 to 18 minutes, until they are tall and golden on top.

Let the biscuits rest on the sheet for 5 minutes, then pull them apart and serve warm with softened vegan butter.