In a saucepan over medium-low heat, simmer the stout beer for 10 minutes or until reduced by more than half. Remove from heat and allow to cool.
In a heavy saucepan over medium heat, combine the cream, milk, sugar, and vanilla. Heat until small bubbles begin to form at the edge. Remove from heat.
In a medium bowl, beat the egg yolks. Add 1 cup of the cream mixture, whisking to combine.
In small amounts, pour the egg mixture into the cream mixture, stirring constantly.
Return to the stove at medium-low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon (approximately 5 minutes).
Remove from heat and strain through a fine mesh strainer into a clean bowl. Cover with plastic wrap, pressing the wrap against the surface of the liquid to prevent a skin from forming.
Refrigerate for a minimum of 2 hours, preferably overnight.
Add the stout beer reduction to the cream mixture and whisk to combine thoroughly.
Pour into the canister of an ice cream freezer, and freeze following the manufacturer’s instructions.
Transfer to an airtight container and freeze until ready to serve.
