Combine 90ml dark rum, 45ml lime juice, 22ml simple syrup, 1 egg white, and 2 dashes Angostura bitters in a shaker without ice.
Dry shake vigorously for 15 seconds to emulsify the egg white and build foam.
Add ice and shake hard for a further 10–15 seconds until well chilled.
Double strain into a chilled Collins glass over fresh ice.
Dust the surface generously with freshly grated nutmeg and serve immediately.
The dry shake before adding ice is essential - it builds a stable, voluminous foam from the egg white without dilution; skipping this step will result in a flat, under-emulsified texture. A reverse dry shake (wet shake first, then dry shake without ice after straining) can produce an even silkier, more stable foam if preferred.
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