Fill a cocktail glass and a shaker with ice. Add gin, Vermouth, Cointreau, and juice of half a lemon to the shaker and shake (or stir, depending on your preference) until well chilled.
Discard ice from cocktail glass and strain into the glass.
Garnish with a lemon peel.
Recipe by The Taste Edit at https://thetasteedit.com/leap-year-cocktail/