Preheat the oven to 180°C, gas mark 4. Grease a 20.5cm loose-bottomed, spring release cake tin and line the base with nonstick baking paper.
Sift the flour, cocoa powder and bicarbonate into a bowl. Stir in the sugar.
Put the butter in a saucepan and melt over gentle heat. Add the Diet Coke to the butter and stir to mix, making sure mixture does not boil. Remove from the heat.
Beat together the buttermilk, eggs and vanilla extract.
Quickly whisk the cola mixture into the dry ingredients, then whisk in the buttermilk mixture. Mix gently but thoroughly.
Pour the mixture into the prepared tin. Bake for about 40 minutes or until a skewer inserted into the centre comes out clean.
Leave to cool in the tin for about 15 minutes, then turn out onto a cooling rack.
To make frosting, place the chocolate and Diet Coke in a saucepan. Cook over a gentle heat, stirring until the chocolate has dissolved. Remove from the heat and stir in the buttermilk. Place the cake on a plate and spoon over the chocolate mixture. Allow to set.