Whisk the eggs and lime zest in a large bowl until well combined and smooth, about 30 seconds. Add the condensed milk and half of the rum and whisk until combined. Whisk in the remaining rum, the nutmeg and bitters until smooth. Strain through a fine-mesh sieve and transfer to a jug. Cover with plastic wrap and refrigerate overnight or up to 3 days-it gets better with time.

Stir the mixture well before serving. Pour into ice-filled glasses and sprinkle with grated nutmeg.

Photograph by Kang Kim