In a bowl over a Bain Marie (the bowl should not touch the hot water underneath) add all the three ingredients and put on medium heat. Stir continuously for about 8 minutes. The egg whites will form a foam and starting to get more firm.
Turn off the heat and transfer the frosting into another bowl. With an electric hand mixer beat the meringue mixture until it gets stiff. It should hold well when you turn the bowl upside down.
Transfer the bowl in the fridge and let it chill for around 30 minutes.
Cover your cake or your muffins.
When you start to use the meringue frosting on your cake pay attention: on the bottom there could be some liquid. Don't worry but try not to use it as it can spoil a little bit the stiff meringue part.