Put 2 cups heavy cream, 1 cup milk, 1/2 cup sugar and a pinch of salt in a sauce pan. Add 1–1/4 tsp root beer concentrate (Schilling’s make this). In a bowl, combine 4–1/2 TBL (that’s a generous 1/4 cup) of corn starch and 3/4 cup milk. Stir with fingertips to confirm there are no lumps. Add cornstarch to sauce pan and heat until steaming, stirring constantly. Lower heat and keep stirring for 5 minutes, or until thoroughly thickened. Strain and chill a couple of hours and freeze in ice cream maker.
NOTE: It struck me after making this that I should have added the flavoring after the mixture came off the heat. Next time.