1. Cream the butter and sugar together until light and fluffy, then add the beaten egg. Mix the cocoa powder, colouring and vanilla together to form a paste then add to the creamed mixture.
2. Sift the flour and add 1 tbsp at a time along with the buttermilk. Blend on high for 30 seconds to get nice and light and fluffy, then switch off the machine and add the soda and vinegar so they start to fizz. Mix again and then spoon into deep cupcake cases (3.8cm height) and bake at 170 °C for 20 minutes until they bounce back to the touch.
3. Cool on a wire rack and in the meantime make the icing by mixing all the ingredients together for several minutes until light and fluffy.
4. Add more milk or icing sugar to change the consistency. You should aim for something that holds its shape without being too stiff. Pipe the icing on top of the cakes using a reusable piping bag and large star nozzle and decorate as required.