Place some crushed ice into the martini glass you will be serving in, and set aside.
Place the 1 cup of crushed ice into a cocktail shaker. Pour in the vermouth and swirl it around, making as much contact as possible with the ice. Using a strainer, pour the vermouth out. Add the gin and stir well to combine.
Remove the ice from your serving glass and add the olive. Using the strainer, pour the gin into the serving glass.