Pre-heat oven to 350. Line 8×8 baking dish with parchment paper or foil (if using foil, spray with a non-stick spray).
In a small saucepan, melt butterscotch chips and butter together, set aside and cool slightly.
In a large bowl, whisk flour, salt, baking powder, and brown sugar together. Stir in butterscotch mixture until combined, mixture will be dry.
Stir in egg and vanilla into mixture until smooth. Add marshmallows and white chocolate chips. Spread batter into prepared pan and bake for 20 minutes until center is set. Cool completely before cutting into squares.
Recipe by Sweet Dash of Sass at http://sweetdashofsass.com/2014/12/marshmallow-blondies/