In a small saucepan, combine the brown sugar and water. Heat gently over medium heat, stirring just until the sugar fully dissolves. Remove from heat, add the lime zest strip, and let steep for 5 minutes only. Remove and discard the zest. Let the syrup cool completely
In a large bowl or pitcher, add the halved grapes and basil leaves. Gently smash with a muddler or wooden spoon just until the grapes release juice and the basil becomes aromatic. Do not pulverize — this must stay green and fresh.
Add the fresh lime juice and 80–100 ml of the cooled brown sugar lime syrup. Stir gently to combine. Taste — it should be tart, herbal, and dry, not sweet.
Add plenty of ice to the pitcher. Pour in the chilled club soda and add the pinch of fine sea salt. Stir once or twice only to integrate without killing carbonation.
Pour into ice-filled glasses and garnish with fresh basil leaves and lime wedges
