Combine all the ingredients in a cocktail shaker with ice.
Shake well.
Strain into a martini glass and garnish with a fresh lime wheel.
Notes
If you only have Triple Sec you can use it here but it is not as complex as Cointreau.I like fresh lime juice here but you can sub in Rose's Lime if you don't have any limes to hand or you'd like a slightly sweeter cocktail. You can also sweeten your drink if using fresh lime with a dash of simple syrup.You can make this a "dirty martini" by adding olive brine but I don't love how that goes with the vermouth I've chosen in this recipe.