Add the food coloring and vanilla essence and beat together until everything is well combined. In a clean bowl whisk together the flour, cocoa powder and baking soda.
In a separate bowl or glass mix together the milk and vinegar, leave for about 5 minutes. Add half of the milk mixture and half of the dry ingredients to the butter mixture.
Mix until combined before adding the second half of the ingredients. Pour into a greased cake tin and bake in a 350f or 180c degree oven for about 35 minutes or until a skewer comes out clean.