
Butter your 9 x 13 x 2 inch baking pan, or line with parchment paper.
Melt butter in a large saucepan over low heat. Once melted, add marshmallows (minus 1 cup) and stir until melted. Remove from heat.
Stir in the vanilla, then add the Rice Krispies and the remaining marshmallows. Gently fold until well combined.
Transfer to the prepared pan and press into an even layer without packing firmly.
Let cool for at least an hour before slicing and serving.