Combine all the ingredients except for the garnish in a Boston shaker half filled with ice. Shake vigorously for 30-45 seconds. Strain the cocktail into half of the shaker, and then discard the ice. Close the shaker again and shake for another 30-45 seconds, without the ice.
Strain into two coup glasses. Let the drink settle for about thirty seconds. Garnish with grapefruit and rosemary.
For the Simple Syrup
Combine the water and sugar in a small saucepan over high heat. Bring to a simmer, and add the rosemary. Simmer for 5-10 minutes, then take off the heat. Let it steep for two hours. Strain, and keep in the fridge for up to two weeks.
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Notes
Make sure when you're shaking, you're going as hard as you can! The better the shake, the better the foam!