Chill your glass. Place a coupe or martini glass in the freezer while you prepare the drink. A cold glass keeps flavors bright and the texture crisp.
Prep the passionfruit. If using fresh fruit, scoop out the pulp and strain out some seeds if you prefer a smoother drink. If using puree, make sure it’s unsweetened so you can control the sweetness.
Build the cocktail. In a shaker, add white rum, fresh lime juice, passionfruit puree, and simple syrup.
Start with 0.5 ounce syrup; you can add more after a quick taste.
Add ice and shake hard. Fill the shaker with ice and shake for 12–15 seconds until the outside is frosty. This chills and dilutes the drink just enough.
Taste and adjust. Crack the shaker, dip a spoon, and taste. If it’s too tart, add a splash more simple syrup, then give it a quick shake.
Strain and serve. Double strain into the chilled glass to catch ice shards and pulp.
The result should be smooth and lightly frothy on top.
Garnish. Add a lime wheel, a few passionfruit seeds for texture, or a mint sprig for aroma.