Fill a martini glass with some crushed ice and set aside.
Set one cup of crushed ice into a cocktail shaker. Pour in a 1/2 oz. of vermouth and swirl it around, making as much contact as possible with the ice. Using a strainer, pour the vermouth out.
Add 2 1/2 oz. of gin and stir well to combine.
Remove the ice from the chilled glass and add 1 olive.
Using the strainer, pour the gin into the serving glass.