Mix the mustard powder with the stout or ale in the bottom of a small pan to make a paste.
Stir in the rest of the beer, butter, and 1 tsp of Worcestershire.
Heat gently until the butter has melted being careful not to burn it.
Slowly add the cheese and stir to melt it but make sure it doesn’t come to a boiling point.
Once it’s melted, taste it to test the seasoning.
Take it off the heat and allow it to cool slightly without solidifying.
Pre-heat a grill to medium and toast the bread on both sides.
Add the beaten egg yolks into the warm cheese until it is blended.
Spoon it onto the toast and serve immediately.