Pour the cornstarch into a medium glass bowl and set aside. Whisk together the cream, milk, and sugar in a medium saucepan and place over low heat. Bring the mixture almost to a boil, but not quite. You just want it hot. Scoop about 1 cup of the mixture into the cornstarch bowl and quickly whisk together very well until smooth. Add the mixture to the saucepan and reduce heat to low. Simmer, while stirring for a few minutes longer until the mixture begins to thicken slightly then remove from heat and add the vanilla and a dash of salt. Divide the mixture into 3 equal portions of about 2 cups each. Add the peanut butter to one, and the cocoa and sugar to the another and stir until smooth, leaving one plain vanilla. Pour each flavor into sealable containers and allow them to cool for at least 30 minutes, then cover and place in the fridge to get completely chilled, at least 4 hours. Once the mixtures are cold, follow the instructions from your ice cream machine for churning them into ice cream, starting with vanilla, then peanut butter, then chocolate so there's no need to clean it out after each flavor (for the peanut butter cup ice cream add the peanut butter cup pieces once the ice cream is finished freezing). Scrape the finished ice cream back into sealable container and store in the freezer for at least 1 hour until solid enough to scoop.