Add all ingredients except the cream soda into a shaker and dry-shake (without ice).
Fill with ice and shake again vigorously.
Strain into a Collins glass over fresh ice.
Top with the cream soda.
Garnish with lime zest.
*Vanilla-bean-infused agave syrup: Stir together 4 oz raw agave nectar and 4 oz water until fully combined. Add 1 vanilla bean, split in half lengthwise, and refrigerate for 12 to 24 hours.