Spread superfine sugar on small plate. Rub lemon wedge halfway around rim of chilled martini or coupe glass. Dip moistened side of glass in sugar to lightly coat outside rim of glass. Set aside.
In cocktail shaker, combine Cognac, Cointreau, and lemon juice.
Add ice and shake vigorously until well chilled, about 30 seconds. Strain into prepared martini or coupe glass and serve.
