Step 1

Place 1 stick unsalted butter in a medium heatproof bowl (or stand mixer bowl fitted with the paddle attachment) and let sit at room temperature until softened.

Step 2

Add 2/3 cup packed dark brown sugar, 1 teaspoon ground cinnamon, 1 teaspoon vanilla extract, 1/2 teaspoon freshly ground nutmeg, 1/4 teaspoon ground allspice or ground cloves, and 1/4 teaspoon kosher salt. Beat with an electric hand mixer on high speed until fluffy and smooth, about 2 minutes.

Step 3

Bring 2 cups water to a boil in a kettle or medium saucepan. Let the water cool slightly so it isn’t actively bubbling, about 30 seconds. Carefully pour the water into the butter mixture. Add 1 cup dark rum and beat with the mixer on medium speed until the butter is dissolved and the mixture is frothy, 1 to 2 minutes.

Step 4

Divide between 4 mugs. Top with whipped cream if desired. Sprinkle with more freshly grated nutmeg. Garnish each mug with a cinnamon stick.