Dry shaking an egg white until frothy gives this take on a Negroni an unexpected creaminess.
Place egg white in a shaker (with no ice); shake vigorously 30 seconds. Fill with ice, rum, Campari, and vermouth. Shake 30 seconds more. Strain into a glass filled with crushed ice; serve, with lime slice.
You can make crushed ice by pulsing regular ice in the food processor. Keep in a large plastic bag in the freezer until ready to use. Tap bag against a hard surface to break up ice before serving.