In a large microwave-safe bowl, add the sweetened condensed milk, white chocolate chips, and peanut butter. Microwave for 30 seconds on 50% power. Stir and repeat microwaving until the chips are melted. Let the mixture cool slightly and then whisk in vanilla, salt, and sour cream.
While the peanut butter mixture is cooling, use a mixer to whip the whipped cream on medium-high speed until soft peaks form, about 2-4 minutes. Whisk in 1/3 of the whipped cream into the peanut butter mixture. Fold in the rest of the whipped cream until just combined, and then fold in the Reese's Pieces and Peanut Butter Cups.
Layer half of the ice cream mixture into an airtight container. Evenly spread the hot fudge over the ice cream, and then layer the remaining ice cream on top. Seal the container and freeze for at least 8 hours before serving.