Lightly grease a 9x13-inch baking pan with butter or non-stick spray. You can also line it with parchment paper for easy removal.

In a large pot over low heat, melt the butter completely.

Add the mini marshmallows to the pot and stir continuously until fully melted and smooth. Remove from heat and stir in vanilla extract.

Quickly add the Rice Krispies cereal and stir until all the cereal is evenly coated.

If using, fold in the extra cup of mini marshmallows for an extra gooey texture.

Press the mixture gently and evenly into the prepared baking pan using a buttered spatula or wax paper—don't press too hard or the bars will be dense.

Let cool for about 30 minutes, then slice into squares and enjoy!