Pre-heat your oven at 180°C /350°F
Cream together the butter and sugar until pale and fluffy, and then gradually incorporate the eggs.
Separately, combine the cocoa, red food colouring and vanilla (either paste or extract) to form a paste, and combine with the egg mixture.
Gradually add the buttermilk and flour, adding a little of each at a time until fully combined.
Add the salt, bicarbonate of soda and vinegar and ensure all ingredients are well mixed.
Pour into baking tray and cook for 25 minutes