Heat milk and cream in a medium saucepan over medium-high heat. Stir until the mixture is simmering, then remove from the heat.
Whisk egg yolks and sugar together until well combined.
Slowly pour 1 cup of the hot liquid into the yolks while whisking at a fast pace.
Stir the yolk mix back to the remainder of the milk in the saucepan, along with the peanut butter. Heat until it thickens, but don't allow it to boil.
Transfer the custard base to an airtight container and leave it in the fridge for at least three hours, preferably overnight.
Churn the custard base in an ice cream maker following the manufacturer's instructions. Once churning is complete, scoop the ice cream into a sealable container and freeze for at least two hours.