Begin by heating your oven to 350°F (180°C). Lightly grease two 7-inch cake tins with non-stick spray, then line the bases with parchment paper circles. Set them aside while you prepare the batter.
In a large mixing bowl, sift together the flour and cocoa powder. Add the sugar, baking soda, and salt, then whisk everything together until evenly combined.
To make the flax egg, stir 1 tablespoon of flaxseed meal into 3 tablespoons of hot water. Let it stand for a minute or two until it thickens to a gel-like consistency.
Add the prepared flax egg to the dry ingredients along with the soy milk, vanilla extract, oil, and vinegar. Stir gently until the batter is smooth and combined. Be careful not to mix more than necessary, as this can affect the texture of the cake.
Pour the batter evenly into the prepared cake tins, smoothing the tops if needed.
Bake for about 25 minutes, or until a toothpick inserted into the center comes out clean with no wet batter attached.
Once baked, leave the cakes in their tins for a few minutes to settle. Then carefully turn them out onto a wire rack and allow them to cool completely before decorating.
When fully cooled, frost the cakes with vegan chocolate frosting or a half batch of vegan chocolate buttercream for a rich and delicious finish.