Stir in 6 cups of the mini-marshmallows, continue stirring until the marshmallows and butter are well incorporated and most of the marshmallows are melted.
Remove from the heat and add 4 cups of the Trix cereal and the Rice Krispies and stir until everything is well combined.
Add the remaining cup of marshmallows and stir gently
Spray a 9x11 pan with non-stick spray or line with parchment paper and press the cereal mixture into the pan with slightly dampened hands.
Sprinkle the remaining cup of Trix cereal on top of the bars and press gently.
Allow to set up for at least 30 minutes, then cut into squares.