In the bowl of a stand mixer fitted with the whisk attachment, whip the heavy cream on high until stiff peaks form. Transfer whipped cream to a separate bowl. Return bowl to mixer.

In a microwave-safe bowl, heat the butterscotch chips in the microwave until melted. Pour into the bowl of the mixer along with the sweetened condensed milk, beating on high until fully incorporated. Add about a quarter of the whipped cream, whipping until mixture is combined. Using a rubber spatula, fold in the remaining whipped cream along with the chopped Reese's Cups.

Heat the peanut butter in the microwave until smooth and stirrable, about 30 seconds.

Pour half of the custard into a 2-quart pan/container. Drop or drizzle tablespoonfuls of half of the peanut butter over the custard. Pour the remaining custard on top, followed by the remaining peanut butter. Swirl the peanut butter into the ice cream using a skewer or a knife. Cover and freeze for at least 8 hours, or until firm.