In a pan over low heat, melt the Butter (1/4 cup). Add the Mini Marshmallows (1 1/2 package) to the melted butter.
While the marshmallows are melting, crush the Oreo® Chocolate Sandwich Cookies (20) and add them to the Rice Krispies® Cereal (5 1/2 cup) in a separate bowl.
Add the Vanilla Extract (1 tablespoon) to the melted marshmallows and stir. Add the Rice Krispie mix to the marshmallows and mix thoroughly.
Grease a 9x9 inch pan and smooth the Rice Krispie mix into all the corners. Let sit on the counter for an hour until set. Cut into pieces.