In a heavy saucepan, over medium heat, whisk together the first four ingredients, milk to salt, and cook, stirring constantly, until thickened. Turn off heat.
Using a small ladle or spoon, add about 2 tablespoons of the milk mixture into beaten egg, whisking egg mixture constantly as the hot liquid is added to prevent the egg from clumping up. I do this a dribble at a time.
Whisking milk mixture constantly, pour egg mixture back into the pot in a very slow, thin stream. When egg is completely whisked in, turn heat back on, to medium.
Whisking constantly, cook egg mixture for just two minutes, then turn off heat and remove pot from burner.
Place the chocolate peanut butter in a small bowl and whisk about half the milk mixture in and combine well, then add this mixture back into the pot, combining all well. Scrape into a bowl (it will be thick), cover with plastic wrap, and refrigerate until completely cold (about 3 hours should do it).
After mixture is cold, remove from fridge and whisk in the light cream. The peanut butter mixture will be very stiff so it's slow-going at first, but bear with it as it will all come together. Once the peanut butter mixture is completely combined with the light cream, whisk in the heavy cream and the vanilla.
Prepare ice cream according to the directions that came with your ice cream maker; I let my Cuisinart ice cream maker run for about 25 minutes and that worked well.
Freeze for several hours to harden. This mixture yields slightly less than one quart.