How to Make It
Preheat the broiler. In a small saucepan, boil the stout over high heat until reduced to 1/2 cup, about 4 minutes. Add the cream and boil until reduced to 3/4 cup, about 4 minutes. Scrape into a bowl and let cool to room temperature. Stir in the cheddar, egg yolks, Worcestershire sauce and dry mustard and season with salt and pepper.
Arrange the toasts on a large rimmed baking sheet. Spread the toasts with a 1/4-inch-thick layer of the cheddar mixture. Broil 4 inches from the heat for about 2 minutes, until the topping is bubbling and browned around the edges. Serve right away.