Typically sours cocktails do include sugar syrup to tone down the harshness of the alcohol and the citrus juice. However, the Amaretto in the Disaronno sour is sweet enough to render sugar syrup unnecessary for the majority of drinkers. If you have a very sweet tooth it is, of course, perfectly acceptable to add sugar syrup to taste. Shake up the drink as instructed, take a small taste and then add in sugar syrup as required. Don't forget to reshake the cocktail if sugar syrup is added.