Preheat your oven to 350°F (175°C). Grease your 9x13-inch baking pan or line it with parchment paper for easy removal later.
In a medium saucepan over low heat, melt the unsalted butter and butterscotch chips together. Stir frequently until smooth and fully combined. Remove from heat and let cool slightly.
In a large mixing bowl, whisk together the melted butter and butterscotch mixture with the packed light brown sugar until well combined. Add in the eggs, one at a time, mixing well after each addition. Then stir in the pure vanilla extract.
In another bowl, whisk together the all-purpose flour, baking powder, and kosher salt until evenly mixed.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Be careful not to overmix; a few lumps are okay!
Using a rubber spatula, fold in the semi-sweet chocolate chips and mini marshmallows until evenly distributed throughout the batter.
Pour the brownie batter into the prepared baking pan, spreading it evenly with your spatula.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. The marshmallows will create a gooey, delicious layer that might look a little underbaked; this is normal!
Allow the brownies to cool in the pan for at least 15 minutes before lifting them out using the parchment paper (if used) and cutting them into squares. Enjoy the magic of your Disappearing Marshmallow Brownies!
