First, heat-treat the egg whites (optional): Warm the eggs in a bain-marie over low heat for 2-3 minutes, whisking constantly. Do this just until the egg whites have become warm but are not cooking. This will kill any bacteria in the eggs, making them even safer to use when raw.

On a stand mixer fitted with the whisk attachment, beat egg whites in a clean, large bowl with the mixer at high speed until foamy, around 2 minutes.

Gradually add sifted sugar spoonful by spoonful, followed by the lemon juice. Beat at high speed until thickened, about 2 - 3 minutes.

The icing at this stage will be at a good consistency for flooding cookies. If you want it stiffer for frosting a cake, or writing on cookies, add more icing sugar until you get the consistency you desire.

Place the icing into a bowl and cover it with cling wrap. Make sure you allow the cling wrap to touch the icing so there is no air in the bowl. This stops it from going hard while you are waiting to use it.

Use it straight away or store it at room temperature for up to 2 weeks. This will be enough to cover 1 large cake.