Add vodka, grapefruit juice, and simple syrup to the cocktail shaker.
Fill the shaker with ice.
Shake vigorously for 10-15 seconds.
Double strain the mixture into the prepared rocks glass using both strainers.
Garnish with a fresh rosemary sprig and grapefruit wedge.
For rosemary simple syrup:
Mix water and sugar in a small pot.
Add rosemary sprigs and bring to a boil.
Reduce heat and simmer for 10 minutes.
Let cool completely and strain.
Notes
Store rosemary simple syrup in an airtight container in the refrigerator for up to 2 weeks.Fresh-squeezed grapefruit juice tastes better than bottled juice.This recipe works with both regular and pink grapefruits.Adjust the vodka-to-juice ratio to make the drink stronger or weaker.