Swiss Meringue Buttercream can seem like a lot of steps, but once you get the hang of it, it will become your new favorite kind of frosting to make. See my tips below if you have any issues.Make sure to start with very clean/dry bowls. Any fat that gets into the meringue can keep it from setting up. If you have a stand mixer, you can heat the egg whites and sugar right in it, over the double boiler. If not, use a mixing bowl and then a hand mixer to make the meringue. Start by getting the butter out of the fridge. Cut up each stick into 1 tablespoon-sized pieces (8 pieces per stick). If you get the butter out right before you make it, it should be the perfect temperature by the time you're ready to add it. It needs to be slightly cold but slightly softened. I've found if I get it out right at the beginning of this process and cut it up, it ends up being perfect.
Next, put a small pot on the stove with 2 inches of water. Bring it to a simmer over medium heat while you separate the eggs.
Separate the eggs into a small bowl one at a time (then if any yolk gets into the bowl, you only have to throw out 1 egg and not all of them). Once you separate an egg white, then pour it into the larger mixing bowl of your stand mixer.*Be sure to not get any yolk into the whites. Any fat in the meringue can keep it from setting up. Add the 2 cups of white sugar to the egg whites in your mixing bowl. place over the simmering water (make sure the water isn't touching the bowl so it doesn't scramble the eggs).
Use a whisk to constantly mix the egg whites and sugar for 6 minutes over the simmering water. At first it will be a thicker paste and it will heat to be a thinner, frothy white liquid. Heat over the double boiler for 6 minutes. After this time all the sugar granules should be dissolved (when placed between your fingers you shouldn't feel any granules - keep heating for another minute if it still has granules after 6 minutes).
Once done, remove from heat. You will transfer your warm mixing bowl with the warmed egg whites and sugar to your stand mixer, and add the whisk attachment. Turn it on to medium-high speed and whisk for 10-20 minutes, until stiff peaks form. The time this takes will depend on how humid it is outside, how hot your house is, and how long it takes the meringue to cool down. Keep checking it every couple of minutes to see if stiff peaks have formed. If after 20 minutes stiff peaks still haven't formed, place the mixing bowl in the fridge for 10 minutes and then continue mixing. Your mixture is probably still too warm.
Once you have stiff peaks, you'll need to change out the whisk attachment for the paddle attachment. This will help beat the butter in and make it smooth.
Once you have the paddle attachment on, you will add the butter 1 tablespoon at a time. At this point, the butter should be slightly softened but still a little cold.
Set the mixer to medium-high speed and add 1 tablespoon of butter at a time, mixing each tablespoon until fully incorporated and smooth. Once mixed in, then add next tablespoon. Continue this until all the butter is added. Keep mixing for a minute to make sure the frosting is smooth. Scrape down the sides if needed and mix for another minute. It should be smooth.*If for any reason your mixture starts to curdle, keep mixing it. It will reincorporate. If that doesn't work, place the bowl in the fridge for 20 minutes and then then try mixing again. After butter is smoothly incorporated, add salt and vanilla (or any other extract you'd like) and mix for another minute.
Mix until fully incorporated. Your Swiss Meringue Buttercream is now ready to frost your cake! It works best to use it right after you make it. If you chill this frosting it will get pretty solid, so you'll need to bring it back to room temp before using it again. I frost my cake and then chill it overnight. Then I get it out of the fridge about an hour before cutting so the frosting has a little time to soften up.
Hope you love this cake and delicious frosting as much as we do!
