In a large pot, combine the unsalted butter and peanut butter chips. Heat over medium heat, stirring frequently, until both are melted and smooth.
Combine with Condensed Milk:
Add the sweetened condensed milk, vanilla extract, and mini marshmallows to the melted mixture. Stir until everything is well combined and the marshmallows are melted.
Prepare the Baking Dish:
Lightly butter or spray a 9×13-inch rectangular baking dish with non-stick spray.
Assemble the Bars:
Spread half of the salted peanuts evenly in the bottom of the prepared baking dish. Pour the peanut butter and marshmallow mixture over the peanuts, spreading it out evenly. Sprinkle the remaining peanuts on top, pressing them gently into the mixture.
Cool and Set:
Refrigerate the dish until the mixture is firm, about 2 hours. Alternatively, you can leave it at room temperature for a few hours if preferred.
Cut and Enjoy:
Once set, cut into bars and enjoy your homemade Payday candy bars.
