Prepare a 15x11 inch jelly roll pan with nonstick spray.
Melt butter in a large microwave safe bowl. Add marshmallows, stirring to coat. Microwave for 1 minute. Remove and stir until smooth. You can heat for an additional 30 seconds if needed. Stir in pumpkin muffin mix. Add cereal, and stir to coat. Press mixture firmly into the jelly roll pan.
Meanwhile, prepare the frosting. In a bowl mix together butter and cream cheese until well combined. Stir in powdered sugar, and mix until fluffy. Mix in vanilla.
Cut the Rice Krispie treats down the center of the pan. Smooth frosting over 1/2 of the treats. Fold over the unfrosted half to make a "sandwich". Allow to set up for about an hour before slicing.