Combine the aged rum, espresso, demerara syrup and the whole egg in a shaker without ice.
Dry shake very hard for 20 seconds to fully emulsify the egg.
Add ice and shake again for 12 seconds until cold.
Double-strain into a chilled coupe.
Grate nutmeg generously over the surface.
The flip is a 19th-century format that modern bars rediscovered in the 2000s, and coffee suits it better than most things. The long dry shake is not optional — under-emulsified whole egg tastes exactly like raw egg, and there is no recovering from that.
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