For the frosting: in a big bowl, using an electric mixer, combine butter ad cocoa.
Slowly add milk and icing sugar until you have the right spreadable consistency.
Add the golden syrup and the vanilla extract, mix well and let sit for at least ten minutes.
You can store in fridge, covered with cling film, for a couple of days; cold it has the texture of chocolate mousse and it’s wonderful by the spoon: I suppose you are going to discover it by yourself.