Yes, you can melt the coconut oil or plant-based butter and marshmallows in the microwave. Place them in a large microwave-safe bowl. Heat on high power for 1 minute, then allow the bowl to rest in the warm microwave for 3 minutes. Then, remove the bowl and stir. It might take a bit of stirring to incorporate the coconut oil into the marshmallows. Just keep stirring until they come together. If needed, you can heat it for an additional 15 seconds, let the bowl rest for a few minutes, then stir again. Check out our Microwave Rice Krispie Treat Recipe for a step-by-step tutorial with pictures.