Make the batter ahead by beating softened butter with demerara sugar, cinnamon and nutmeg until smooth — it keeps in the fridge for weeks.
Put a heaped teaspoon of the batter into a warmed mug with a strip of orange peel and a strip of lemon peel.
Add the dark rum and allspice dram and stir to loosen the batter.
Top with hot coffee, stirring as you pour so the butter emulsifies rather than floating.
Grate nutmeg over the surface.
From Victor Bergeron's Trader Vic bars, where the spiced butter batter was kept in a crock behind the service station. The butter is what separates this from every other hot rum coffee: it emulsifies into the drink and gives it a weight nothing else replicates. Make the batter in quantity; it is the only slow part.
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