STORAGE: Keep buttercream at room temperature if using the same day. If not, transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes. Add the egg whites to the bowl of a stand mixer. Fit the mixer with the whisk attachment and whisk on high speed for about 4-5 minutes. The mixture will become thick and glossy and will begin to cool. Begin by filling a medium sized pot with water to about ¼ of the way. Let it come to a gentle boil. Add 1 tbsp of butter at a time while the mixer is on high speed. At first your egg whites will deflate and look like the butter is causing them to split, but don't panic! It will come together. It takes a couple minutes for the butter and meringue to become best friends. Separate your egg whites from your yolks in a large, very clean, glass or metal mixing bowl. We only need the egg whites for this recipe so you can set the yolks aside for a different recipe. If you need ideas there are loads on the internet but my go recipe for using egg yolks is custard. Add your sugar to the egg whites and use a whisk to mix them together. Put your bowl of egg whites and sugar mixture on top of the pot of boiling water. Use a whisk to whisk as your egg whites gently cook. This is called the double boiler method. Make sure the bottom of the bowl is not touching the water in the pot otherwise you'll end up with scrambled eggs. Once the frosting has come together, has gone pale in colour (if you're making vanilla) and is nice and fluffy then it's done!. Once all your butter has been added, add some vanilla extract, and if you're making chocolate buttercream add the cocoa powder too. Mix on low speed to mix in and then mix on high speed for another 5 minutes. Continue gently whisking for about 3-4 minutes. What you want to do is whisk until the sugar is completely dissolved. The best way to check that it's dissolved is by running it through two fingers. If you can't feel the sugar granules then it's time to take it off the stove. If you find your frosting is too aerated and has too many air bubbles then mix the frosting in your stand mixer on the lowest speed for about 10 minutes and it will become perfectly smooth. IS IT SAFE TO EAT? Yes. The process of whisking the egg whites with the sugar over a double boiler helps gently cook the egg whites making it safe to eat.