Line an 8x8-inch pan with aluminum foil and spray lightly with oil. Set aside.

In a large pot over low heat, stir sweetened condensed milk, butterscotch chips, and marshmallows until chips and marshmallows are melted and the mixture is smooth. Stir in peanut butter and vanilla. Fold in choppped peanuts.

Transfer to the prepared pan and smooth out the top. Refrigerate for at least 2 hours before cutting. Store in refrigerator.