1/2 cup Guinness or dark stout/ale, room temperature
3/4 tsp Worcestershire sauce
8 oz white cheddar cheese, shredded
4 slices rye bread, lightly toasted
In a medium sized saucepan over medium heat, melt butter. Whisk in flour, cooking for about 2-3 minutes. Whisk in mustard powder, cayenne, salt and pepper. Pour in stout and Worcestershire sauce. Whisk until thickened.
Lower heat and add cheese, whisking constantly, until smooth. Remove from heat.
Spread cheese sauce evenly on toast and heat under broiler until the edges are browned and the center is bubbly. Serve immediately.