Slice your loaf in half and trim accordingly. Pop under the grill to toast. Remove and leave to cool.
Meanwhile, get on with your cheese mix. Melt the butter in a pan over a low heat. When it's foaming slightly add the flour. Cook for a minute or so until it smells biscuity but not brown. Pour in your guinness and mix well to eradicate any lumps. Add mustard powder, cayenne and the worcestershire sauce. Season with a little salt.
Gradually and gently mix through your cheese, in batches is best. Once all smooth and incorporated, pour into a dish and pop in the fridge to harden. Once firm spread all over your toast. Pop under the grill until blackened, bubbling and awesome.