Preheat the oven to 400 degrees. Spray a 1-quart souffle dish with nonstick cooking spray and dust generously with granulated sugar. Place the souffle dish in the freezer to chill.
Stir 1/4 cup of the superfine sugar with the lime juice in a small bowl until the sugar is dissolved. Set aside. Using very clean, dry beaters, beat the egg whites by hand or with an electric mixer until foamy. Gradually add the remaining 1/4 cup of superfine sugar and the salt while beating. Continue beating until the whites hold a soft peak. Gently fold in the lime juice mixture and the zest.
Spoon the souffle mixture into the chilled souffle dish. Wipe the rim with your fingers or a damp cloth to ensure proper rising. Put the dish on a baking sheet and place in the oven immediately. Bake for 7 to 9 minutes, until the souffle is well risen and lightly browned on top. Serve and eat immediately.