Line an 8x8-inch baking dish with parchment paper and lightly grease.
In a medium saucepan, melt the butter over low heat.
Add the butterscotch chips and stir constantly until smooth and melted.
Remove from heat and let cool for 5–7 minutes.
Fold in vanilla extract and a pinch of salt.
Gently stir in mini marshmallows until evenly coated.
Pour mixture into prepared pan and press lightly into an even layer.
Refrigerate for 2–3 hours or until firm.
Cut into squares and serve chilled or at room temperature.