Chef John's Hot Buttered Rum

Ingredients (12)

• + 8 tablespoons unsalted butter, at room temperature add to pantry
• + ½ cup firmly packed dark brown sugar add to pantry
• + ¼ teaspoon pure vanilla extract add to pantry
• + ½ teaspoon ground cinnamon add to pantry
• + ⅛ teaspoon ground ginger add to pantry
• + ¼ teaspoon ground nutmeg add to pantry
• + ⅛ teaspoon ground cardamom add to pantry
• + 1 pinch ground cloves add to pantry
• + 1 pinch salt add to pantry
• + 6 tablespoons heavy cream add to pantry
• ✓ 1 ⅛ cups dark rum
• + 4 ½ cups boiling water, or as needed add to pantry