Preheat the oven to 350 degrees F. Line a 10x15-inch rimmed baking sheet with parchment then lightly grease with cooking spray. Set aside.
In a large bowl, whisk together the flour, sugar, brown sugar, cocoa, baking soda and salt. Stir in the melted butter, eggs, and vanilla. Mix just until combined (be careful not to overmix). Spread the batter evenly in the prepared pan.
Bake for 16 minutes or until a toothpick inserted in the center comes out clean. Be careful not to over-bake.
Remove from the oven, sprinkle the marshmallows evenly over the top of the brownie. Carefully place the hot pan back in the oven and bake for 2 more minutes or until the marshmallows are soft. (We’re not melting them, just softening them.) Remove from oven and allow to cool completely before glazing.