From Elizabeth Andoh, An American Taste of Japan [Isbn 0-688-04369-0]: combine water and sugar in a small saucepan over low heat, stirring, until the sugar melts, and simmer the syrup for 5 minutes. Stir in the mirin and remove from heat. In a separate bowl, mix I tablespoon of the syrup with the powdered green tea, then add that mixture to the syrup in the saucepan, and stir until evenly mixed. Add milk and half and half, pour into an ice cream freezer, and freeze according to manufacturer's instructions.