Step 1: Prepare Your Baking Dish - Start by greasing a 9x13-inch baking dish or lining it with parchment paper.
Step 2: Melt the Butter and Butterscotch Chips - In a large mixing bowl, combine the softened unsalted butter and the butterscotch chips. Microwave in 30-second intervals, stirring in between until melted and smooth.
Step 3: Add the Peanut Butter - Stir in the creamy peanut butter until fully incorporated.
Step 4: Season It - Sprinkle in the salt and mix well, adjusting to taste.
Step 5: Fold in the Marshmallows - Gently fold in the mini marshmallows, reserving ½ cup for topping.
Step 6: Transfer to the Baking Dish - Pour the mixture into the prepared baking dish and spread evenly.
Step 7: Add the Final Touch - Sprinkle the reserved mini marshmallows on top, pressing them down slightly.
Step 8: Chill - Refrigerate for at least 2-3 hours until fully set.