Pre-heat the oven to 180°C/Gas Mark 4. Line two 12-hole cupcake tins with paper cases and set aside.
Put the cocoa, food colouring and vanilla extract in a small bowl and mix well.
Put the butter and sugar in a large bowl and use a hand-held mixer to beat them together. Add the yolks and beat for one more minute.
Add in the cocoa mixture and continue to mix for another minute. Slowly pour in the buttermilk, followed by the flour and baking soda.
Fill each paper case with three tablespoons of batter and bake for about 15 minutes, or until a toothpick inserted in the centre comes out clean.
Allow to cool in the tins on a wire rack for 15 minutes before icing.