Put the marshmallow cream, heavy cream, light corn syrup, and vanilla extract in a medium, microwave-safe bowl.
Cook on high for 30 seconds and stir. Return to the microwave and cook on high in 15-second intervals, stirring each time, until smooth.
Video
Notes
Makes 1 1/2 cups.Allow the marshmallow sauce to cool, then store it in an airtight container in the fridge for up to one week. Reheat briefly in the microwave.