Italian meringue is made by beating egg whites until they reach soft, fluffy peaks, then slowly streaming in boiling sugar and beating the mixture until it is thick and glossy. A basic meringue, also known as a French meringue, is made by beating granulated sugar into egg whites until the mixture reaches soft peaks. The hot sugar syrup used to make Italian meringue essentially cooks the egg whites as it is incorporated. This means that you don’t need to cook or bake the meringue before using it. It also means that the meringue is going to be a lot more stable and less likely to deflate or weep than a simple meringue is.
When making an Italian meringue, it is important to add the boiling sugar syrup very slowly, so that you don’t accidentally cook your eggs by overheating them. It is also important to keep the mixer (or your arm, if you are buff/brave) working on a medium-low speed while you work, so that the syrup is incorporated consistently without flying off the whisk attachment or beaters of your mixer. Once the syrup has been incorporated, the meringue is beaten at high speed for several minutes until it has cooled down. Unlike a simple meringue, you cannot overbeat an Italian meringue, so there is no need to worry about that. Once the meringue has cooled down somewhat (some recipes will call for it to be warmer or closer to room temperature for use), it can be used in a variety of recipes.
You’ll frequently see Italian meringues called for as pie toppings, especially for Lemon Meringue Pie, and in desserts like Baked Alaska. Italian meringues are used in some macarons and the meringue can also be piped and baked to make plain meringue cookies. One of the most popular uses for Italian meringue, however, is making rich, buttery Buttercream Frosting, where butter is beaten in to an Italian meringue until the mixture is transformed into one of the most decadent ways to top of a cake or cupcake.