Combine a rounded ¼ cup of cornstarch and ¾ cup whole milk in a large saucepan. With your fingertips stir until all the cornstarch is dissolved. Add 1½ cups extra-strength coffee, 1½ cups whole milk and ½ cup sugar. Stir over high heat until mixture is steaming. Continue cooking one medium to low heat until mixture thickens completely: 4-5 minutes. Strain mixture in fine wire mesh, then stir in 1½ tsp pulverized coffee (a commercial coffee grinder will do this at the finest setting).
Cool custard and freeze in an ice-cream freezer.
NOTE: I used decaf coffee, and to make the extra-strength just used half as much water as I normally would for the amount of coffee.