Preheat oven to 350°F. Line a 9x13 inch pan with parchment paper.
In a large, microwave safe bowl, heat butterscotch chips and butter in 20 second intervals until they are melted together, stir until smooth.
Stir in the flour, brown sugar, baking powder, eggs and vanilla extract until well mixed. Fold in the mini marshmallows and chocolate chips.
Spread the mixture evenly in the prepared pan. Bake for 20-25 minutes in the preheated oven until the edges pull away from the pan and the center is set.