Melt the butter over low heat in a large pot, stirring regularly to prevent the butter from burning.
Once the butter has melted, add the marshmallows and vanilla extract to the pot. Stir the mixture until the marshmallows are completely melted, stirring frequently to prevent burning.
Remove the pot from the heat and fold in the cereal. Make sure each piece of cereal is coated with the marshmallow mixture.
Transfer the mixture to the prepared baking pan. Gently spread the mixture to fit the pan and press it down lightly so you have an even layer.
Put the chocolate in a microwave safe bowl or dish, and heat it on high for 30 second intervals until melted.
Pour the melted chocolate over the Rice Krispie treats and then transfer the pan to the fridge for 45 minutes to let the chocolate harden, or let them set for 2 hours at room temperature instead.
Cut the treats into pieces and enjoy!
Notes
Nutritional values are an estimate and will vary depending on the exact ingredients used.
Store these treats in an airtight container at room temperature for up to 3 days or in the fridge for up to 7 days.