Melt two tablespoons butter in a heavy skillet over medium heat. Place baguette slices into melted butter and toast until golden. Turn baguette slices and toast until the other side is golden too.
Remove toasted bread slices from pan and place on four plates.
Add two tablespoons butter in the skillet. Once the butter is melted sprinkle two tablespoons of flour. Whisk until the flour is incorporated into the butter and starts to turn golden. Turn heat down to low and slowly whisk in beer and milk until smooth.
Add mustard, paprika, cayenne, Worcestershire sauce continuing to whisk the sauce. Add cheese ½ cup at a time whisking until melted before adding more cheese.
Once cheese has been added and is melted, remove from heat. Add egg yolk and combine.
Top toast slices with cheese sauce and garnish with chives.