Preheat the oven to 400°F (200°C). Lightly grease a baking sheet or line it with parchment paper.

In a small bowl, whisk together the soy milk and apple cider vinegar. Set aside for 60 seconds to allow it to curdle.

In a large mixing bowl, whisk or sift together the flour, sugar, baking soda, and salt. Add the cold, diced butter and use your fingertips or a pastry cutter to work it into the flour mixture until it forms coarse crumbs.

Pour the curdled soy milk mixture into the dry ingredients and stir until a soft dough forms. If using raisins, fold them in at this stage. Transfer the dough to a lightly floured surface and gently shape it into a round loaf, roughly 8 inches (20 cm) in diameter. Avoid overmixing to maintain a tender texture. Place the loaf on the prepared baking sheet.

Using a sharp knife, cut a deep cross, approximately ½ inch (1 cm) deep—across the top of the loaf. If using oats and brown sugar as toppings, sprinkle them evenly over the surface.

Bake for 35–40 minutes, or until the loaf is golden brown and sounds hollow when tapped on the bottom. Transfer to a wire rack and allow to cool for at least 20 minutes before slicing.