Gather together and measure out all the ingredients before you begin.
Spray a 13x9 inch dish with non-stick cooking spray. Set aside.
Melt 3 tablespoons butter in a large pot over medium low heat.
Once the butter is melted add the marshmallows to the pot.
Stir until marshmallows are melted, stirring constantly, 3-4 minutes. Low and slow produces the best results.
Once the marshmallows are completely melted, add one teaspoon of vanilla extract. Stir to combine with the melted marshmallows.
Add rice Krispy cereal to the pot.
Turn off heat and stir to combine everything. Stir until all the rice cereal is coated with the marshmallow mixture.
Spoon the mixture into the greased 13x9 inch dish.
Use a piece of parchment paper to spread the cereal mixture out in the dish in an even layer.
Allow treats to cool for one hour.
Cut into 24 squares to serve.
Store treats in an air tight container.