Preheat oven to 350°F (180°C). Lightly grease and line a 9 x 5 inch (23 x 13 cm) loaf pan with baking paper.
In a small bowl or jug, combine soy milk and vinegar. Let sit for 5–10 minutes to curdle.
In a large mixing bowl, whisk together pumpkin purée, brown sugar, oil, vanilla, and the curdled soy milk mixture until smooth.
In a separate bowl, sift together the flour, baking powder, baking soda, salt, and your chosen spices.
Add the dry ingredients to the wet and stir until just combined — do not overmix.
Pour batter into the prepared pan, smooth the top, and sprinkle with pepitas if using.
Bake for 55–65 minutes, or until a skewer inserted into the center comes out clean or with a few moist crumbs.
Let cool in the pan for 10–15 minutes, then transfer to a wire rack to cool completely.