Italian meringue lends itself to a large range of uses. Whipping a hot (240°F!) sugar syrup into foamy egg whites doesn’t just make it the most stable of the meringues—it’s also safe to eat without additional baking, which is why it’s traditionally used to make buttercream frosting, or “Italian Buttercream.” Italian meringue is also the most involved of the meringues because it requires a little bit of sugar cookery, but once you understand some meringue basics and have a good thermometer, it’s as easy as meringue pie.
In a heavy-bottomed saucepan, add your sugar. Pour in just enough water to give it the consistency of wet sand. For a four egg white meringue, you'll need about one cup of sugar and a half cup of water.
Begin cooking the sugar on high heat, stirring only until it comes to a boil. Once it reaches a boil, stop stirring. Make sure to wipe down the sides of the pan with a wet pastry brush to clean off any sugar granules. This prevents small granules from caramelizing on the side of your pan and the sugars from crystallizing in mixture as it cools.
The moisture content and hardness of a sugar syrup can be accurately gauged by its temperature. In this case, we're looking for the "soft ball stage," the temperature at which a small amount of sugar dropped into a bowl of cold water will form a ball that holds together but is still malleable. Use a candy thermometer to monitor the temperature of your sugar. 240°F is what we're after.
While the sugar syrup heats, wipe the bowl of a stand mixer with a lemon wedge. (If you prefer, you can also add a 1/2 teapsoon of lemon juice or cream of tartar directly to your egg whites). Place four egg whites into the bowl and set your mixer to medium speed. You want the egg whites to reach soft peaks by the time the syrup has come to temperature. To check for soft peaks, pull the mixer head out of the meringue. It should form gentle peaks that very slowly collapse back into themselves.
When your sugar reaches 240°F, carefully, carefully, remove it from the stove. (Hot sugar is just as dangerous as fryer oil, so use caution!) Very gradually stream the hot sugar syrup into your egg whites as they continue to whip on low to medium speed.
Once you've added all of the syrup, increase the mixer speed to medium-high. Continue mixing until the meringue reaches the desired peak. Soft peaks are often used to aerate mousses, for example, while a stiff peak is best for buttercream.
Pro-tip: To clean out the pot you used to cook the sugar, fill it with water and bring it to a boil. This should dissolve any sugar that's hardened in the pan.