First chill a 10-ounce martini glass. Then, in a 16-ounce pint glass, build the lime and orange with 16 mint leaves. Muddle until mint is aromatic and citrus are juicy. Shake vigorously with fresh ice until shaker is frosted. Add rum, Grand Marnier and simple syrup and then strain into chilled martini glass. Finish with bitters. Float mint leaf on top.