In a microwave-safe mug, microwave the butter and marshmallows on high for 20 to 30 seconds. Be careful here as the marshmallows will puff up quite a bit and might overflow.
Carefully remove the mug from the microwave, add in a drop or two of vanilla extract, and stir until smooth. Add in the crispy rice cereal and stir until well combined.
Dig in while warm and you’ll feel just like a kid again!