Preheat the oven to 350℉.
On medium heat, heat butter until golden. Set aside to cool. In a large bowl, beat brown butter with brown sugar then beat in vanilla, cinnamon, salt and eggs.
Add dry ingredients to wet and mix until just combined.
Pour batter into a 8×8 parchment-lined baking pan.
Top with marshmallows and chocolate. Bake for 20-25 minutes.
Allow to cool for 15 minutes before removing from pan and serving!