Firstly, you'll need to reduce the chickpea water to 1/2 cup measurement. Heat your chickpea water in a saucepan, bring to the boil, then reduce to a simmer for 5 minutes.
Once reduced, leave this in the fridge for an hour to cool. This step can also be done the day before.
To make your sugar syrup, combine the water and 3/4 cup of sugar in a sauecpan and stir to form a thick paste. Bring to the boil, then reduce to a simmer for 10 minutes. You want your sugar to be 116c on a candy thermometer.
Whip up your chickpea water and cream of tatar using an electric whisk and clean, dry mixing bowl. Once youre mixing has increased in size and formed soft peaks, add in the remaining 1/4 cup sugar whilst continuing to whisk.
Once your sugar syrup is ready, gradually drizzle this in small quantities at a time as you whisk. You're mixture should be beautifully glossy and form peaks.
Chop the butter into small chunks. Continue beating the mixture with your electric whisk and add in a little of the butter at a time, whisking for a few seconds in between. The meringue will flatten as the butter is added, but continue to wisk and your mixture will thicken when all of the butter has been fully incorporated.
Add in the freeze dried berry powder (could also use food colouring) and beat to evenly distribute.
You're now ready to spread or pipe this onto your cakes. You can store the buttercream in the fridge, but will need to whisk again before using.