Wrap the ice cubes up in a clean towel and pound with a rolling pin to crush.
Fill a martini glass with 1 cup of the crushed ice and set aside to chill.
Add the remaining 1 cup of crushed ice to the bottom of a Boston-style cocktail shaker. Pour in the gin and Amaro Montenegro and stir until chilled, approximately 15 seconds.
Discard the ice from the martini glass and strain the gin mixture into the serving glass. Garnish with grapefruit peel and serve immediately.