Line a 10x15-inch jelly roll pan with foil and spray lightly with cooking spray, set aside.
In a large pot, melt the butter over low heat. Add in the marshmallows and stir until they are just melted. Remove from heat and stir in the dry cake mix. Add the Rice Krispies and sprinkles, mix well.
For the filling: In the bowl of a stand mixer (or you can use an electric hand mixer), using the paddle attachment, beat the butter until smooth. Add the powdered sugar, cream, vanilla and pinch of salt. Mix until well combined and smooth (will be fairly thick)
Cut the Rice Krispie layer in half, vertically (so you'll have two rectangles about 5 x 7 1/2-inches). Spread the buttercream filling evenly over half of the Rice Krispie layer. Place the other half on top of the frosting to make a giant "sandwich", pressing down lightly.
Cut into bars. Store in an airtight container (if there's any left).
A jelly roll pan is not quite as big as a large cookie sheet. If you only have the large cookie sheet, go ahead and use that, line it with foil and spray it, but when you press the Rice Krispie mixture into the pan, just don't press it all the way to fit the pan, leave about 2 to 3 inches on one end.