Preheat oven to 425*. Pour milk into a small glass; stir in lemon juice. Let sit at room temperature for 10 minutes.

In a medium bowl, comine flour, corn flour, xanthan gum (if using), baking powder, and baking soda. Cut vegan butter spread into flour mixture until mixture resembles fine crumbs.

Using a fork, stir in enough reserved soured milk so the dough leaves the sides of the bowl and rounds up into a ball. (Too much milk makes the dough sticky; not enough makes it dry. You may have some left over.) Turn dough onto a lightly floured surface and push together. Roll or pat dough into a 3/4-inch-thick circle and cut with a floured 2 1/2-inch biscuit cutter. Place biscuits on a dry baking sheet, 1 inch apart for crusty sides or touching for softer sides.

Bake on center rack for 10-12 minutes, or until golden brown. Transfer to cooling rack. Serve warm.