Thickening the milk well reduces the formation of ice crystals in kulfi. Make sure the mixture is properly cooled before you pour it in the moulds. Close the lid of the mould tightly to avoid the formation of ice crystals in the kulfi. Also, make sure to freeze it at the lowest temperature possible. For best result, empty the freezer as much as you can before freezing it so that the chances of ice crystal formation are very low.