Butter a 9x13 inch baking pan or line with parchment paper.

In a large saucepan, melt the butter over low heat.

Add the marshmallows and stir constantly until completely melted and smooth.

Remove the saucepan from the heat and stir in the vanilla extract.

Add the puffed rice cereal and fold gently until all cereal is evenly coated with the marshmallow mixture.

Transfer the mixture to the prepared pan and press down lightly with a buttered spatula or wax paper.

Allow the treats to cool completely at room temperature for at least 30 minutes.

Cut into squares and serve.