Prepare a 9x13" baking pan with cooking spray, or line with parchment paper (also spray the paper to prevent sticking) for easy removal and cutting.
In a large microwave safe bowl, heat the large marshmallows and butter together in 30 second intervals, stirring between each time. The marshmallows and butter should melt together within about 2 minutes.
As soon as the marshmallows are melted, add the vanilla and stir.
Then quickly add the mini marshmallows and cereal. Stir until the cereal is completely coated.
Press the cereal mixture into the prepared pan evenly. Don't press too tightly or the bars will be hard. Press firmly, but also gently.
Allow the bars to set for about 20 minutes at room temperature, then cut into bars and serve.
Store in an airtight container at room temperature.
The calories shown are based on the bars being cut into 15 pieces, with 1 serving being 1 bar. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**