Combine the yolks and half the cream in a medium-sized bowl. Whisk until foamy.
Add the remaining cream, the milk, the sugar and the peanut butter to a large saucepan and heat over low heat, whisking to dissolve the sugar and peanut butter. Once the mixture is smooth, crank the heat up to medium-high and allow the mixture to come to a boil, whisking occasionally. As the mixture heats, it will thicken.
Gently drizzle the hot, thick peanut butter mixture into the yolk mixture, whisking continuously to prevent the eggs form scrambling.
Once everything is combined, pour the mixture back into the saucepan and heat gently over low heat, whisking constantly. Clip on a sugar thermometer (or the probe of a multimeter) and heat the mixture until it reaches a temperature of 80°C (175°F).
Immediately remove from the heat and pour the peanut butter custard (which it now is) trough a medium-meshed strainer to make sure it's completely smooth. Whisk in the vanilla, then press a piece of plastic wrap directly onto the surface of the custard. Allow the custard to come to room temperature before placing it in the fridge to chill for at least an hour.
Churn in your ice cream maker according to the manufacturer's instructions, or freeze the mixture 'manually' by placing it in a freeze-proof container in the freezer and stirring vigorously every half hour until it has frozen (this may take up to 3 hours, depending on the temperature of your freezer).
Once churned, place the ice cream into a clean bowl, plop in dollops of the hot fudge sauce (make sure it's only slightly warm!) and gently fold it into the ice cream. Transfer the ice cream to a freeze-proof container and place in the freezer to firm up for at least 4 hours or overnight.
Recipe by The Tough Cookie at https://thetoughcookie.com/2014/11/17/peanut-butter-ice-cream-chocolate-fudge-swirl/