Coat the bottom and sides of a 9x13-inch baking dish with salted butter (unsalted works too). Side Note: For easier removal, you can line the dish with a bit of parchment paper if desired.
Brown the butter in a medium saucepan over medium heat. Stir the butter often once it has melted. It will start to foam at the top and you will see the color of the butter change to brown and start smelling nutty. This process only takes about 5 minutes and the turn happens quickly, so watch carefully.
Remove ½ cup of mini marshmallows from the bag and set aside until later.
Add the remaining marshmallows to the brown butter, stirring constantly until melted and smooth, about 3-4 minutes. Side Note: You may need to put the saucepan over low heat to help the marshmallows melt.
Once melted, add the vanilla extract and stir to mix well.
Immediately add the rice krispies and stir until well combined.
Add the ½ cup of mini marshmallows and most of the mini chocolate chips and stir gently to quickly combine.
Immediately spread the mixture in the prepared baking dish pressing it firmly into an even layer, using parchment paper to protect your hands, if desired.
Sprinkle the few remaining mini chocolate chips evenly on top.