Preheat oven to 220°C (430°F). Line a baking tray with a lightly grease piece of parchment paper. Set aside.
In a medium-size mixing bowl, whisk soy milk with apple cider vinegar. Set aside 10 minutes until it thickens to a buttermilk-like consistency.
In another large mixing bowl, whisk together flours, baking soda, and salt if desired.
Make a well in the center and pour the vegan buttermilk.
Stir the dough with a spatula at first then, when it becomes difficult to work the dough with the spatula, use your hands to knead the dough, adding more flour if needed to form a ball.
Knead the dough for 1 or 2 minutes maximum, until you are able to make a ball. If the dough is very sticky, simply add more white flour (up to a maximum of 1/3 cup) but be careful, you don't want a dry dough or the bread will be very dense and dry. A stickier dough results in a moist, elastic bread and that's what you want.
Transfer the dough ball onto the prepared baking tray and flatten the dough into a thin disc of a maximum 3 cm (1.5 inches) in height. This is extremely important or the bread won't raise at all.
Cut a deep cross (1 cm deep, 0.4 inches) on top of the bread and sprinkle/pat some extra white flour on top of the bread.
Bake at 220°C (430°F), middle rack, for 20-30 minutes or until the outside is crusty.
Transfer to a cooling rack for 1 hour before slicing.
Store the bread at room temperature for up to 3-4 days, wrapped into a clean towel to keep its moisture.
It can be eaten plain or toasted with jam or mashed avocado. Or it can be served as a quick side to soup and salads