Melt the butter in a pan, stir in the flour, and let this cook together until it smells biscuity but is not browning. Add the mustard powder and cayenne pepper, stir in the Worcestershire sauce and the Guinness, then gently melt in the cheese. When it’s all of one consistency, remove from the heat, pour out into a shallow container, and allow to set. It will keep happily in the fridge for a couple of weeks in this state.
Take each piece of bread and toast on both sides. Allow to cool just a little, then cover one side with the rarebit mixture to about 1cm thick – if you find that it doesn’t spread with ease, press it on with your fingers. Put on a baking sheet and place under the grill until golden and bubbling – grilling to just beyond your comfort threshold, to allow the flour to cook out.
