Please read over the recipe and instructions fully before getting started as well as the above blog post which contains more detailed instructions, images, and tips.
Preparation (can be done a day ahead)
Start by reducing the aquafaba, add it to a saucepan and bring 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.
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.
Add the freeze-dried strawberries to a food processor and blitz for a few minutes until they turn into a powder consistency. Allow the powder to settle in the food processor for a minute before opening the lid. Then pass the powder through a fine-mesh sieve to remove any seeds. If you're using strawberry powder you can skip this step.
Add 200g of the caster sugar and 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, prepare the aquafaba. Take a large clean bowl, and make sure it's completely dry and free of any grease. Add the aquafaba and cream of tartar to a large bowl and whisk on high speed for 5 minutes until soft peaks form.
Slowly add in the remaining 50g of caster sugar, one dash at a time while continuing to whisk and incorporate the sugar into the aquafaba. Continue whisking for another 3 minutes.
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.
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 be thick like buttercream, continue to whisk on low for 5 minutes.
Add the vanilla extract and whisk again on low for another 2 minutes. Then add the strawberry powder, one tablespoon at a time, until all of it has been incorporated into the buttercream.
Storage and Use
This meringue buttercream is best when used fresh. You can also store it 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, re-whipping if needed.
Aquafaba: drain 1-2 cans of unsalted chickpeas, use only the liquid part.
How to fix Italian meringue buttercream: 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. Different brands can vary in water/fat content so if your buttercream isn’t turning out how it should and you have followed all instructions and measurements to a T you can try adding more vegan butter whilst whipping until creamy.