If the meringues are cracking, they are likely cooling too quickly. Once the timer goes off, check once and then leave in the warm oven to slowly cool and dry out further (they will still be chewy inside). This is an incredibly flexible recipe that may be doubled or tripled by simply weighing egg whites and doubling the amount of sugar (if 150 ml egg whites, then add 300 gr sugar)—no need to adjust the vinegar or vanilla. Caster sugar is superfine sugar. Cane sugar may be whirred in a food processor to assimilate the finer texture of caster sugar.