In a small bowl, combine 2 tablespoons softened unsalted butter, 2 tablespoons packed brown sugar, and 1 teaspoon granulated sugar. Stir until smooth and fully incorporated.

Add spices and vanilla

Mix in 1 teaspoon ground cinnamon, 1 teaspoon ground nutmeg, and a pinch of ground cloves. Add 1 teaspoon vanilla extract and stir until the spice mixture is evenly distributed and the paste is creamy. Taste and adjust sweetness or spice as desired.

Heat water

Bring 6 ounces (about 3/4 cup) of water to a gentle simmer — do not boil vigorously.

Assemble drink

Place the prepared butter-sugar-spice paste in a pre-warmed heatproof mug. Pour in 2 ounces dark rum. Slowly add the 6 ounces hot water, stirring continuously until the butter mixture is completely melted and the drink is homogenous.

Garnish and serve

Taste and adjust if needed (a touch more sugar or a dash more nutmeg). Garnish with a lemon peel or orange twist. Optionally top with a dollop of whipped cream and a light dusting of grated nutmeg. Serve immediately while hot.