1. Make sure to have a completely clean and dry bowl when you start your process, any fat or liquid at all in the bowl will stunt the protein development of the albumen (egg white protein) and you will not have a proper meringue at the end.
2. Start whipping egg whites on medium low speed in the mixer by themselves (no sugar or butter yet) until the whites are foamy and opaque. It should look like the head of beer.
3. Increase to medium speed and slowly start adding the sugar until all the sugar is incorporated.
4. Once all the sugar is in, increase the speed of the mixer to high and whip until the mixture is shiny and stiff. You now have a meringue. You know when your meringue is done when you pull out the whip, hold it horizontal, and if you have what looks a “sparrow’s beak” on the end of the whip.
5. Replace the whip, turn the mixer on medium and start adding the butter a bit at a time. Once all the butter is incorporated, turn the mixer on medium speed and let it whip until the butter has completely emulsified into the meringue. When you first add the butter, your meringue will break down and look like cottage cheese, but will continue to emulsify into a smooth buttercream. Depending on the weather, the buttercream could take anywhere from 5 to 15 minutes to form. When the buttercream is ready, the mixture will be homogeneous and smooth…and tasty.
6. Remove the buttercream from the bowl and place in an airtight container. Buttercream can be kept at room temperature for a few days or in the fridge for a week or two, but always use soft buttercream when icing a cake. To warm up the buttercream, put it back in the mixer using a rubber spatula. Begin remixing the buttercream using the whip attachment on high speed while applying direct heat to the outside of the bowl with a propane torch that can be found at any hardware store.
Note: Don’t worry about using raw egg whites in your buttercream, the sugar cooks the egg whites and makes them perfectly safe to eat. If you are still uneasy about this, use pasteurized egg whites.