Bring cream, milk, peanut butter, corn syrup and vanilla to a simmer in a saucepan over medium-high heat, whisking occasionally until combined. Whisk yolks and sugar in a bowl until pale (4-5 minutes), then, whisking continuously, pour in hot cream mixture and whisk to combine. Return custard to pan and stir continuously over medium heat until mixture thickly coats the back of the spoon (5-6 minutes). Transfer to a bowl and refrigerate, whisking occasionally, until chilled (2-3 hours). Freeze in an ice-cream machine (be careful not to overchurn or the mixture will split) and serve within a couple of hours in chilled cups, topped with mini marshmallows, popcorn and crumbled pretzels.