Preheat your oven to 350°F (175°C). Lightly grease and flour a 9-inch round or 8x8-inch square cake pan.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, and baking soda until thoroughly combined.
In a separate small bowl or liquid measuring cup, mix together the soy milk, vanilla extract, applesauce, oil, and vinegar.
Pour the wet into the bowl with the dry .
Stir with a spatula just until the batter is smooth and no dry streaks remain. Be careful not to overmix.
Immediately pour the batter into the prepared cake pan and spread evenly.
Bake for 26–28 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Allow the cake to cool in the pan for at least 15 minutes before inverting it onto a wire rack to cool completely before frosting.
Do not overmix the batter.
Mix only until the wet and dry are just combined to avoid developing gluten, which can make the cake tough.
Bake the cake immediately after mixing. The leavening reaction between the vinegar and baking soda starts right away, and you want to capture that lift in the oven.
