In a microwave-safe bowl, heat the chocolate chips and butter for 45 seconds on high powder. Stir to melt and combine, then whisk in the brown sugar, cinnamon, cayenne, vanilla, and salt.
Bring the water to a boil in a small sauce pot. Remove from the heat and pour in the rum. Stream in the chocolate mixture, whisking constantly, until smooth.
Divide the cocoa into two large mugs. Garnish with lots of whipped cream, cinnamon powder, and cocoa powder, as desired.
Keyword chocolate, christmas, cocktails, drinks, holidays, hot buttered rum, hot chocolate, rum