Put the espresso in a tall glass and stir in simple syrup to taste. Add ice cubes and the lemon twist. Fill to the top with sparkling water. Serve immediately.
To make simple syrup, combine equal parts sugar and water in a small saucepan and bring to a boil over high heat, stirring once or twice to dissolve the sugar. Remove from the heat and let cool. Refrigerate, covered, indefinitely.