Add all ingredients into a shaker with ice and shake until well-chilled.
Strain into a chilled cocktail glass.
Garnish with a rosemary sprig.
*Grapefruit-rosemary-infused vodka: Add 1 L vodka, 1 peeled and sliced grapefruit and 2 sticks of rosemary into a Mason jar and sous vide for 2 hours at 130 degrees Fahrenheit, starting the timer once the bath hits the desired temperature. (Make sure you place the Mason jar in the bath before starting the sous vide, otherwise the rapid heating could break the jar.) Strain once the jar is cool enough to handle. Store in the refrigerator for up to a few weeks.