To make the butter: In a small saucepan over medium heat, combine the brown sugar, butter, honey, cinnamon, nutmeg, cloves, and salt. Heat, stirring occasionally, until the butter is melted and the mixture is combined, about 5 minutes. Remove from heat and stir in the vanilla.
To make each cocktail: combine the hot water with about a quarter of the butter in a mug, and then add vigorously to combine.
Add in the rum and stir to combine. Taste, and add additional butter if you like it a bit sweeter or spicier.
Top with a large dollop of whipped cream and fresh grated nutmeg.