Return to the egg whites and turn the mixer to medium speed. With the mixer on medium speed, slowly pour the sugar syrup into the egg whites, aiming for the side of the bowl rather than the whisk and egg whites. When all the syrup is added, turn the mixer to medium-high and whisk until the icing becomes shiny, thick, and holds a firm peak. This should take around 10 minutes — don't whisk any longer than that, or the icing will become too thick to spread and pipe!