Preheat the oven at 350°F.
Grease a 9 inches round baking pan.
In a bowl mix sugar, flour, baking powder, baking soda and salt.
Add the sour cream, the oil, the egg and the vanilla extract.
Beating quickly by hand or with a food processor add slowly the hot coffee ( or water).
Pour the batter in the baking tin and bake for about 30- 35 minutes.
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.
Spread a generous amount of frosting on the cake when it's completely cold.