SHAKE all ingredients with ice and fine strain into chilled glass.
The oomph of rum, the sourness of grapefruit and the richness of maple syrup, all aromatised by bitters.
My (Simon Difford) version of an old drink created by Herb Smith and popularised by his friend Oscar at the Waldorf, New York City. The drink was named in honour of the Duke of Windsor and his bride, formerly Wallis Warfield Simpson. The four 'W's stand for Wallis Warfield Windsor Wallop.