Add ingredients to shaker. Shake over ice.
Strain over 1 ounce seltzer in a Collins glass filled with cubed ice.
Express a lemon peel and float on top and serve with 2 long straws.
* For the avocado syrup, blend 1½ cups sugar, 1½ cups water and 1 ripe avocado. Pour into a bottle and refrigerate for up to 1 week.