Butter a 9×13″ baking dish, set aside. You can also line the pan with foil and butter that for easy removal.
For the bottom layer, melt butter in a large saucepan over low heat and add marshmallows. Stir until they begin to melt, however don't let the marshmallows melt all the way. Then stir in the fruity pebbles. Then you’ll press the fruity pebbles mixture into the baking dish. I found it easiest to push down the mixture with a spoon coated in butter or wax paper.
For the top layer, melt butter in a large saucepan over low heat and add marshmallows. Once almost melted, stir in the cake mix. Then stir in the rice krispies cereal. Mix until it is completely coated with the marshmallow mixture. Sprinkle in half of the sprinkles and stir until combined.
Next, pour the cake batter rice krispie treats on top of the fruity pebbles treats and top with more sprinkles if desired.
Let cool before cutting. I find using a plastic knife to cut works best.
If you would like to make only fruity pebbles treats, simply double that layer and omit the cake batter layer.