Add the butter into a large pot over medium heat, and let it fully melt.
Add 8 cups of mini marshmallows, frequently stirring until melted. Once melted, remove from heat.
Lightly spray a rubber spatula with nonstick spray (to prevent everything from sticking to it) and use it to fold in the fruity pebbles cereal, rice krispies cereal, and the remaining 2 cups of marshmallows. Stir until the marshmallows are well-coated.
Now, transfer the mixture into your prepared baking dish. Press it into an even layer.
Let the the bars cool to room temperature before cutting and serving.
Notes
Take the melted marshmallows off the heat- As sugar cooks, it hardens, so take the melted marshmallows off the heat as soon as they melt to keep your treats soft and chewy.
Spray the spatula with cooking spray- To keep the marshmallow mixture from sticking to the spatula, oil the spatula as you work.
Don't let the mixture stick to your hands- Press the mixture into the pan with a sheet of waxed or parchment paper on top, or use wet (not dripping) hands to press them firmly into the pan.
Let it cool fully- Don't try to cut the Fruity Pebbles Treat before they cool as they will be very gooey and hard to separate.