Place the water, brown sugar, butter, and all spices in a large crock pot. Stir well to dissolve the sugar.
Cover the slow cooker for 3+ hours on HIGH heat or 6+ hours on LOW heat.
When ready to serve, whisk in the rum and heavy cream. Taste, and add a pinch of salt or extra cayenne if needed. Ladle into mugs and serve warm.
To make on the stove top, simply combine all ingredients except for the rum and heavy cream in a large pot and heat over medium-low heat. Bring to a gentle simmer, stirring frequently, until the sugar is dissolved. Turn off the heat, and whisk in rum and heavy cream when ready to serve.You can top with whipped cream and extra nutmeg if desired!