Lightly grease a 9x13-inch pan with non-stick cooking spray. (Optional: line pan with parchment paper before spraying for easy removal.)
In a large microwavable bowl, add the marshmallow fluff and butter. Microwave for 1-2 minutes in 30-second increments until completely melted.
Briefly stir the melted mixture to combine. Immediately mix in the salt and vanilla extract. While it's still warm, quickly add in the rice krispie cereal and stir until the cereal is completely coated.
Press the cereal mixture into the greased pan and set aside to cool. Once cooled and fully set up, cut into squares and serve.
Notes
Spray the bowl and spatula with non-stick spray to prevent the marshmallow mixture from sticking too much.
Line the pan with parchment paper before spraying to easily remove the treats from the pan after they set up.
Store in an airtight container at room temperature.