Place mint leaves, lime wedges, and sugar (or syrup) in the bottom of the glass. Muddle gently to release flavors.
Add blackberries if using, muddle again lightly.
Fill glass with ice cubes or crushed ice.
Add white rum and lime juice. Stir to mix.
Top with sparkling water, stir gently again.
Garnish with a mint sprig and lime wedge. Serve immediately.
Notes
Muddle mint gently—over-muddling turns it bitter. Use simple syrup for quicker mixing. Try blackberries for a festive look and berry flavor. Crushed ice chills the drink faster and keeps it lively.