At least a day before serving, prepare the base for this ice cream: Pour half of the beer into a small, nonstick skillet, and bring to a simmer over medium heat. Simmer until the beer is reduced by half, about 10 minutes. Remove from the heat, and add the remaining 6 ounces of beer and the vanilla, stirring to combine. Set aside.
While the beer is cooking down, combine the cold water and ice in a large bowl.
Whisk together the sugar, salt, and egg yolks. Pour the egg yolk mixture into a large saucepan, and turn the heat to medium-low. Slowly whisk in the cream, and cook, stirring constantly, until the mixture thickens. (Don’t rush this part or your ice cream will be lumpy!) The mixture should be 180°F when it is fully cooked. If you don’t have an instant thermometer, you can use the old-fashioned custard test: Dip the back of a wooden spoon in the mixture, then drag your finger across the custard coating the spoon. If it leaves a clean trail, your custard is done.
Pour the custard through a fine mesh strainer set over a wide shallow bowl. Whisk in the beer mixture, and set the bowl in the prepared ice bath. Set aside until the mixture has cooled to 80°F, then cover with plastic wrap, and refrigerate for 8 to 12 hours.
Freeze the mixture in an ice cream maker according to the manufacturer’s instructions, then freeze until firm before serving. The ice cream will keep in the freezer for 5 days.