Preheat oven to 180•c and line a 12 cup cake tin with liners.
Using a stand mixer or electric whisk, cream butter and sugar together until smooth and pale in colour. Add in the soya milk and mix briefly to incorporate.
Sift the flour and salt into a medium size bowl and add to the creamed butter and sugar in three batches, mixing briefly after each addition.
In a small bowl, mix ground flaxseed with water to form a gel or flax 'egg'. Add vanilla extract, food colouring and cocoa powder to the flax egg and mix well with a fork until incorporated. Add this to the cake batter and process briefly to combine.
In a small cup, mix apple cider vinegar with baking soda. Allow it to bubble up, then add the cake batter and process until fully incorporated.
Pour batter into liners until each is 3/4 full and bake for 15-18 minutes, until a toothpick comes out clean. Allow to cool for 5 minutes in the tin before placing cakes on a wire rack to cool fully.