1. In a medium saucepan, warm the milk, water, sugar, and vanilla bean until it’s almost to a boil.
2. Remove from heat and add the pieces of white chocolate, whisking until they’re melted. Strain through a fine-mesh sieve into a large bowl set within a larger bowl of ice. (Rinse and air-dry the vanilla bean, and reserve it for another use.)
3.Stir the mixture until cool.
4. Freeze in your ice cream maker according to the manufacturer’s instructions.