Remove one of the cooled Rice Krispie Treat batches from the pan and place it onto a large serving tray.
Apply a thick layer of buttercream onto the top of the giant Rice Krispie and then place the second batch of Rice Krispies on top and add a thick, even layer of frosting on top.
Use a knife to create a rectangle on the upper left part of the rectangle and fill it with blueberries.
Add raspberries to create the red stripes.
Add the buttercream into a piping bag with a tip and add dollops between the lines of raspberries.
This can be made ahead of time and stored in the fridge, but make sure to serve at room temperature.