10largefresh egg whites or liquid egg whites (300 g),Fresh whites are slightly better in terms of firmness, but liquid egg whites are pasteurized with no salmonella and safer to eat. I've used both and can't really tell the difference.
6sticks(3 cups/680 g) unsalted butter**,cut into tablespoon-size pieces and cool, but not cold
1TBLSpure vanilla extract
Remove butter from the refrigerator and cut into tablespoon-sized pieces. Leaving the butter out while you work on the next few steps gives it just the right amount of time to soften slightly (touching it will leave a tiny indentation) while still ensuring that it is quite cold.
Wipe the bowl of an electric mixer and utensils with paper towel and a little lemon juice or vinegar, this removes any grease which would make the meringue not rise. Make sure bowl and utensils are completely dry.
Place egg whites, sugar and salt in the stand mixer bowl, then place bowl over a simmering (not boiling) pot of 1" of water (don't let the water touch the mixing bowl), whisking gently, but constantly until the egg temperature reaches 160°F on a candy thermometer (the temperature necessary to kill salmonella in raw eggs).
Remove from heat. With whisk attachment of mixer, whip on medium to medium-high speed until the meringue is thick, glossy, and the bowl cools to room temperature (about 10 minutes or so on medium or high speed). If the bowl is slightly warmer than room temperature, do not add the butter yet or it will melt, just continue whisking for another couple minutes and check again.
Switch to paddle attachment and, with mixer on medium speed, add in the butter one tablespoon at a time, until incorporated.
Turn the mixer up to medium or high speed and keep beating until the frosting is light and fluffy. Important note: During this process, the gorgeous meringue will suddenly deflate and will look soupy with butter chunks and/or curdled. Don't worry, it's not ruined! Just keep mixing and eventually it will come together again and become beautiful smooth frosting (this can take about 10-15 minutes).
Add the vanilla. Turn to low speed for about 5 minutes to get rid of any air pockets to make it silky smooth.
**Unsalted Butter Use the brand of unsalted butter that you like the flavor best. Not all butters taste the same. I read that Lurpak unsalted butter is more white, if you want whiter frosting. I like Plugra brand, but I have used regular American unsalted butter (like Darigold) when I don't have European butters available.
Piping on cupcakes This was the perfect amount for me to pipe simple swirls for 36 regular sized cupcakes. This recipe is easily halved, too!
Cool, good. Warm, bad. This type of recipe does not hold up well under heat. So I would not suggest using this for cupcakes/cakes being left outside in hot weather.
To store: Room temperature General rule that this can be left out at room temperature for 2 days in airtight container. Cooler area of the house is better than near warmer areas. Think of butter + heat = melt.
To store: Refrigerator Can keep in the refrigerator in an airtight container for up to one week. Remove from refrigerator and let it sit for at least 1 hour to bring to room temperature. With paddle attachment in stand mixer, beat on medium speed until smooth (5-15 minutes). It may break (liquid separates from solids into a soupy mess). Either wait a little longer and beat again, or just keep beating until it's brought back together. It's not ruined, just keep mixing! Again, if there are little air pockets in the nearly finished frosting, turn speed to low for 5 minutes to get it silky smooth.
To store: Freezer Can freeze for up to 6-8 weeks. To thaw, place on counter overnight. With paddle attachment in stand mixer, beat on medium speed until smooth (5-15 minutes). It may break (liquid separates from solids into a soupy mess). Either wait a little longer and beat again, or just keep beating until it's brought back together. It's not ruined, just keep mixing! Again, if there are little air pockets in the nearly finished frosting, turn speed to low for 5 minutes to get it silky smooth.
To store: Frosted cakes Store frosted cakes/cupcakes with Swiss meringue buttercream in an airtight container either at room temperature for 2 days or refrigerator for longer, but always leave out on counter to bring to room temperature before serving.