Line a loaf tin with baking paper making sure that the paper extends over the sides and ends of the pan.

In a large non-stick pan, combine the sugar, cocoa, buttermilk and food coloring. Stir to combine it all then bring the mixture to the boil.

Stir it constantly but don't scrape the sides of the pan.

Keep heating and stirring until the mixture reaches at least 170C(238F). This takes quite a while so be patient.

Now reduce the heat to a simmer and leave it to simmer away. Don't touch it though. No stirring!

Remove the pan from the heat and let it cool for about 30 minutes then add the butter (still no stirring) and allow it to cool to 43C (110F).

Transfer the mixture to the bowl of an electric mixer and add the vanilla. Beat it until the fudge loses its shine.

Pour the mixture into the lined tin and leave it to set. I put mine in the fridge.

Cut the firm fudge into squares to serve.