In a heavy saucepan, whisk together the water and the cornstarch and simmer the mixture over moderate heat, whisking, for 2 minutes. Be careful: if you simmer for too long, the cornflour will lose its stabilization powers.
Add the milks, the salt, and the sugar, heat the mixture over moderately low heat, whisking, for 1 to 2 minutes, or until the sugar is dissolved, and remove the pan from the heat.
Cool the mixture in the fridge, or in a freezer bag in an ice bath, until it reaches 40°F (5°C).
Stir in the Guinness. You can use a hand blender to mix it thoroughly if you've got one.
Add the mixture to your ice cream maker and churn until it reaches a soft serve type texture (usually 15 to 30 min). Transfer the mixture to a plastic freezer container and place into the freezer for at least an hour.
