*Cook's Note: Make sure to have a completely clean and dry mixing 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. The results could be disastrous.
Start whipping egg whites slowly in the mixer until foamy. Increase the speed of the mixer and slowly start adding the sugar until all the sugar is incorporated. Once all the sugar is in, increase the speed of the mixer even more 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 horizontally, and if you have what looks a "sparrow's beak" on the end of the whip.
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 high and let mix; depending on the weather, the buttercream could take anywhere from 5 to 15 minutes to form. You will know when it has formed when you hear the motor of the mixer start to slow down and whine a little bit; also, when you first add the butter, your meringue will break down and look weird and this is what you want. When the buttercream is done, the mixture will be homogeneous, consistent, and tasty.
Remove the buttercream from the bowl and transfer to an airtight container. Buttercream can be kept at room temperature for a few days or in the refrigerator for a 1 to 2 weeks, but always use warm buttercream when icing a cake. To warm up the buttercream, put it back in the mixer using the whip or the paddle, and apply direct heat with a propane torch you can find at any hardware store.
Professional Recipe: This recipe was provided by a chef, restaurant or culinary professional and makes a large quantity. The Food Network Kitchens chefs have not tested this recipe in the proportions indicated and therefore cannot make any representation as to the results.