Line an 8×8-inch pan with parchment paper and spray with nonstick spray, leaving some overhang for easy lifting.
In a large pot over low heat, melt the butter completely. Stir in cinnamon until fragrant, about 30 seconds.
Add marshmallows and stir until smooth and melted. Remove from heat and mix in vanilla and salt.
Quickly fold in the Cinnamon Toast Crunch cereal until fully coated. Gently stir in crushed Oreos, white chocolate chips, and sprinkles.
Transfer to the prepared pan and lightly press into an even layer using greased spatula or fondant smoother. Avoid pressing too firmly to keep the bars soft and chewy.
Sprinkle more rainbow sprinkles on top for that birthday look.
Let cool at room temperature for at least 1 hour, then lift from the pan and cut into large squares.