Measure the Rice Krispies cereal and place them into a large mixing bowl.
In a large saucepan, melt the butter over medium heat. Stir in the vanilla extract.
Gradually add the mini marshmallows to the melted butter, stirring continuously until completely melted and smooth. Add them in increments, allowing them to melt before adding more.
Once the marshmallow mixture is smooth, pour it over the Rice Krispies. Gently stir until all the cereal is evenly coated.
Add sprinkles and mix lightly a few more times.
Transfer the mixture into a lightly greased 9×13-inch baking dish. Gently press it down using the back of a spatula. Do not press too firmly, or the bars will become dense.
Sprinkle additional toppings if desired and allow the mixture to cool for at least 30 minutes.
Cut into serving sizes and serve.
These treats can be eaten immediately, but they hold their shape better and taste best once fully cooled.
