Over medium heat in the double boiler, heat up the heavy cream, subsequently adding the ground marijuana. Simmer uncovered for 1.5 hours. Next, allow the mixture to cool slightly and pour it into an airtight container, placing the container in the refrigerator for 3 - 4 hours. Take out of the refrigerator and mix in sugar and vanilla, only beating them in for around a minute. Any excess beating or mixing can result in lumpy whipped cream. Place on your favorite desert, in hot chocolate or simply eat with a spoon as it is.