Shake the ingredients with ice and strain into a chilled cocktail glass.
In about 1921, the cocktail muse inspired the great Constantino Ribalaigua Vert of the El Floridita Bar in Havana when he added fresh grapefruit juice and maraschino liqueur to the Daiquiri. The result is ambrosia. The drink was made as a frozen drink at the famous Floridita and was named Papa Doble, or "Papa's Double," after Ernest Hemingway. The original recipe didn't call for any sugar, just a touch of the maraschino liqueur, and it is always reprinted that way out of respect to Papa; Hemingway had an aversion to sugar. But you can be sure that for the average customer at the Floridita, the Simple Syrup was part of the recipe.