Prepare the brown butter – place a stick of salted butter in a small sauce pan over medium low heat. Allow the butter to melt completely. Whisk occasionally until the top of the melted butter looks foamy. Once it looks foamy, whisk constantly until you start to see small brown flecks at the bottom of the pan and immediately reduce the heat to low.
Pour in the mini marshmallows and stir constantly until the marshmallows are completely melted. Turn off heat and pour in the cereal. Stir quickly and mix until cereal is evenly coated.
Pour into baking dish and press down into an even layer. It may be helpful to use a piece of parchment paper for this step. Allow to harden completely before cutting into squares or rectangles.