Prepare a 9X13 pan, or an 8x8 pan with parchment paper.
Brown the butter over medium high heat. Allow the butter to melt and bubble until it is an amber color.
Add in the marshmallows. Continue to stir them until they are mostly melted
Add in the rice crispies and stir until combined.
Add the vanilla extract or vanilla bean paste into the sweetened condensed milk stirring until combined. Then pour the milk mixture into the pot. Stir until combined.
Scoop the mixture into the prepared pan. Allow to "gel" together in the fridge or on the counter for 30-1hr. Slice and enjoy.