1. In a shallow dish, spread sugar for rimming glass.
2. Using a citrus zester with channel knife, make a lemon twist by puncturing the lemon with channeling knife and firmly dragging it around the fruit to create a long, thin spiral. Set aside.
3. From the same lemon, cut a wedge. Run lemon wedge around the rim of a martini glass, then rim the glass with sugar.
4. To a cocktail shaker 2⁄3 full of ice, add vodka, Cointreau, lemon juice, and agave. Shake vigorously until well chilled, about 5 seconds. Strain into rimmed martini glass and garnish with a lemon twist.