Remove the pan from heat, then whisk in 2 tablespoons of the milk until smooth. Whisk in 2 more tablespoons of milk. Add the rest of the milk in three parts, whisking until smooth in between additions.
1 cup milk
Stir in the beer, dijon mustard, cayenne and salt. Return the pan to the heat and stir until thick and bubbling, about 2-3 minutes. Turn off the heat and remove the pan from it.
Add the cheese a handful at a time, whisking after each addition until melted through and smooth. Add more salt and cayenne to taste. Garnish with a pinch of cayenne and chopped parsley.