Like your hot cocoa extra-rich? Increase heavy cream to 3 cups, and decrease milk to 1 cup.
Bring first 4 ingredients to a low boil in a saucepan over medium heat, whisking often. Whisk in cream, and cook, whisking often, 3 to 5 minutes or until bubbles begin to form around edges of pan. Remove from heat, and stir in vanilla. Serve immediately with Our Best Homemade Marshmallows.