Melt 1 tablespoon butter in the microwave. Pour into bundt pan and spread up the sides using a pastry brush.
In a large pot or Dutch oven, melt remaining butter (4 tablespoons) and mini marshmallows, stirring with a wooden spoon.
As soon as the mixture melts and combines, add Rice Krispies, working fast to cover all cereal in marshmallow. Turn burner to low or "keep warm" setting.
Place 1/2 of the mixture into the bundt pan and press into the bottom and up the sides, creating a well.
Spoon marshmallow creme into the well.
Spoon remaining Rice Krispie mixture over the fluff, covering all holes and pressing down firmly.
Allow to cool to room temperature before inverting and transferring to a serve plate. If you remove the cake too soon, it will not have set and will fall.
Decorate using desired colored frosting and sprinkles.
If you've tried this recipe, come back and let us know how it was!