2) Stir in the warm beer by degrees, until you have a thick but smooth sauce. Add the grated cheese and stir until melted. You should now have a thick paste.
3) Mix in the mustard and Worcestershire sauce and season well with black pepper. Remove from heat, add egg yolk and mix.
4) Roll out for required portions, cut half of the mix in single-portion sizes, wrap in greaseproof paper and freeze.