Makes approximately 8 cups.

Special Equipment
Stand mixer with whisk and paddle attachments.
Candy thermometer

Ingredients
1/2 cup cold water
2 1/4 cups granulated sugar
1 cup aqua faba (the liquid drained from a can of chickpeas or other mildly-flavored beans — see note above about reducing the liquid before you start if you need a firm buttercream)
20 to 24 ounces (about 3 to 3 1/2 cups) 100% palm oil shortening(*), at cool room temperature
1 tsp vanilla extract

Directions
Place the aqua faba into the bowl of a stand mixer that has been fitted with the whisk attachment.
Put sugar and water in a non-reactive saucepan, and whisk or stir until blended. Place the pan over medium low heat until the sugar is fully dissolved. Remove any grains of sugar on the sides of the pan by either a) heating briefly with a lid on the pan, or b) brushing the insides of the pan with a heat-safe pastry brush dipped in cold water.

Attach your candy thermometer so that the tip is submerged in the sugar syrup, but is not touching the bottom or sides of the pan, and start the mixer on low speed. At this point, you may increase the stove temperature to medium, or even medium-high if you’re prepared to pay close attention to it.

When the sugar syrup reaches the soft ball stage (about 234ºF), turn your mixer speed to high. This is not a magic temperature, and if you’re using reduced aqua faba, you may want to start a little later. The goal is to get your meringue to very stiff peaks before you add the sugar syrup to it. Exactly when to turn up your mixer will vary depending on your mixer, your aqua faba, and how high the heat is under your saucepan, so start with this, and make adjustments as necessary for future batches.

If you are able, take note of the progress of the meringue as your sugar syrup rises in temperature, and turn the temperature down or up to try to get the timing right.
When your sugar syrup reaches 248ºF, remove it from the heat and verify that your meringue has reached very stiff peaks. You can wait a couple of minutes at this point if you need to, but try not to wait too long.

With the mixer still running on high speed, slowly pour in the sugar syrup, trying to carefully hit the sweet spot in between the spinning whisk and the side of the bowl (otherwise you might splatter sugar syrup everywhere…but don’t worry if you do, just keep going!). Sugar syrup is very hot! Be careful!Meringue with sugar syrup added.
Your meringue will probably increase in volume as you add the sugar syrup, and when all of the syrup is added, it should be thick and fluffy. At this point, stop the mixer and change to the paddle attachment.

Turn the mixer back on high, and keep beating until the meringue has cooled off to room temperature, which can take several minutes. During this time, the meringue will become glossy, and may decrease a bit in volume. Touch the sides and bottom of the bowl to ensure that the meringue is completely cool before continuing — otherwise the heat of the meringue will melt your fat, and you’ll get soup, not buttercream! If you’re impatient, you can speed up this process by putting an ice pack against the bottom and/or sides of your mixer’s bowl.

Now it’s time to add the shortening, vegan butter, or other fat. With the mixer still running, add small pieces to the meringue, one at a time. If you’re using shortening in a tub, scoop out spoonfuls and toss them in; if you’re using margarine or other products that come in a stick, cut it into small pieces first, and toss them in one by one. You will probably lose some more volume as you start adding fat, but that’s expected.

Note: As you are adding your fat, your buttercream may “break.” (If you’re not sure what this means, don’t worry — if it happens to you, you will know! If your buttercream looks curdled, just keep beating with the paddle, and it should come back together again. If it still won’t come together, especially if it’s a warm day, you may need to cool it down for a bit and try again. Place the buttercream (bowl, paddle, and all) into the refrigerator for a few minutes, then try again.

When all of the fat has been added, turn your mixer to medium low and and add vanilla extract or other flavorings. Stop the mixer and scrape down the bowl with a rubber spatula a couple of times, to ensure that your flavorings are well-mixed.

Italian Meringue Buttercream stores extremely well in the refrigerator or freezer, so don’t be afraid to make it in advance, or keep some on hand for a rainy day. Just make sure to take it out and allow to fully return to near room temperature before use. Then, beat it in your stand mixer if you need to restore the texture.

(*) A Note on Fats: You may be wondering if there are other options beyond 100% palm oil shortening. Surely there are, but as of this writing, I have not fully explored the options. See the notes above for a more in-depth discussion about fat options. Margarines and butter substitutes that come in a tub are probably not suitable, because their moisture content is much higher than products that are sold in stick form. I will update this recipe as I discover other options that work.