Incredible vegan pistachio buttercream that's silky, smooth, and not overly sweet. This frosting is based on Italian meringue buttercream and is ideal for cupcakes, sheet cakes, and layer cakes.
Reduce the aquafaba: Add it to a saucepan, and bring it to a boil. Reduce the heat and allow it to simmer until it has reduced to half a cup in volume. After 5 minutes of simmering, check the volume by pouring the liquid into a measuring cup. If there's more than half a cup of liquid continue to simmer until it reduces to the required half cup (118ml).Pour the reduced aquafaba into a jar and refrigerate. This can be done the night before or at least one hour in advance of making the recipe.
Make the sugar syrup: Add 200g of the sugar and the water to a saucepan, gently stir and allow the mixture to sit for a minute until the sugar dissolves into a paste.Place the saucepan on medium-high heat and allow the mixture to come to a simmer and bubble. DO NOT STIR throughout the entire process. Place your candy thermometer into the syrup and continue to simmer until the mixture reaches 116°C (240°F) - about 10-12 minutes. Meanwhile, move on to the next step.
Whip the aquafaba: Take a large clean bowl, and make sure it's completely dry and free of any grease. Add the aquafaba and lemon juice or cream of tartar to a large bowl and whisk on high speed for 5 minutes until soft peaks form.
Slowly add the remaining 50g of caster sugar to the aquafaba, one dash at a time while continuing to whisk and incorporate the sugar into the aquafaba. Continue whisking for another 3 minutes.
Drizzle in the syrup: Once the sugar syrup has reached the correct temperature begin to add it to the whipped aquafaba. Slowly drizzle in a little at a time and continue to whisk until the mixture is incorporated.Whisk the meringue mixture for another 10-15 minutes until it has cooled to room temperature.
Add the butter: Start slowly adding the chunks of butter to the meringue, one at a time, allowing the mixture to incorporate for about 5 seconds between each one. The mixture will flatten as you add the butter, just continue to add the butter as it will stiffen when the full amount of butter is incorporated.Once all the butter has been added the mixture will be thick like buttercream, continue to whisk on low for 5 minutes.
Flavor the buttercream: Add the pistachio butter and whisk again on low for another 2 minutes.
Storage: You can store this Italian meringue buttercream in an airtight container in the fridge. It will harden when chilled, so before use, allow it to come to room temperature until it comes back to a creamy consistency.
Notes
Aquafaba: Drain 1-2 cans of unsalted chickpeas until you have the required amount of liquid. Use only the liquid part.
Sugar: Use organic if US-based to ensure the sugar is vegan.
Vegan Butter: I recommend Flora plant butter or Violife block.
I highly recommend using a candy thermometer. However, if you can’t wait to get one you can do something that’s called a softball test, you can read more about it on cooksinfo.com.
If your buttercream looks split even after adding all of the vegan butter to it, this can often be fixed by adding a little more butter.