Do this step first to avoid running around to get it together when the time comes! Place a medium or large sieve over a mixing bowl.
Add the soy milk, vanilla extract, and salt to a sauce pan. Mix to combine and leave to simmer on medium heat. It will simmer pretty quickly, in a few minutes.
In a small or medium-sized mixing bowl, combine the sugar, cornstarch, and Just Egg.
Mix with a whisk until evenly combined.
Once the milk mixture has started to simmer, slowly pour half of it into the Just Egg mixture while stirring with a whisk.
Next, slowly pour in all of the mixture from the mixing bowl back into the pot with the remaining milk mixture.
Whisk continuously on medium heat to cook and thicken the mixture. It should be thick enough to see the streaks of the whisk in it as you mix.
Turn off the heat. Scoop the mixture from the pot into the sieve you prepared earlier. Using a rubber spatula, press it through the sieve to get rid of any lumps in the mix. Don’t forget to scrape the outside bottom of the sieve to get any of the mix that collects there.
Cover the mixture with plastic wrap to prevent a skin from forming on the surface. Allow it to cool down to room temperature. The amount of time this takes will vary. You can also speed up the process by popping it into the fridge or freezer for a few minutes. Don't let it get cold though, because it can cause the buttercream to split when you add the room temperature butter to it.
Add the custard to the bowl of a stand mixer. Mix with a whisk attachment to loosen it up.
Add in the *room temperature* butter a tablespoon or two at a time while mixing at medium or high speed. Allow each addition of butter to fully incorporate before adding the next one.
We added some violet food coloring to whiten the yellow mixture a bit.
To flavor the buttercream we added in heavy cream and strawberry powder at this point. We also added a little bit of red food coloring for extra color. Mix it until it's well combined. And then add it to a piping bag. If your frosting is a little runny, you can refrigerate it for a few minutes to make it hold up a little better.