Spray 9x9 pan with cooking spray, then line pan with parchment paper on the bottom and up two sides.
Melt butter in microwave.
Mix with brown sugar until smooth. Add egg and vanilla, and mix until smooth. Add flour, baking powder, and salt, and mix until all the flour is incorporated into the mixture. It will be thick.
Fold in butterscotch chips, marshmallows, and chocolate chips until just combined. Mixture will be stiff, but you’ll be able to fold them all in.
Use a spatula to scrape into pan.
Bake for 20-25 minutes, until sides are lightly browned and center is just firm.
Notes
Blondies are done with the edges are golden and crispy, and the middle is just set (i.e., it does not wiggle when you nudge the pan). The best way to know for sure is to check with a cake tester or toothpick. Stick your tester in the middle of the brownie pan. If it comes out with just a few crumbs on it, the blondies are done. If you hit a melted chocolate chip, and your toothpick comes out with chocolate on it, that’s fine too. You may want to test in another place to make sure.