Soften the vegan butter at room temperature so it is still firm, but pliable
Prepare the apples by peeling & coring them
Add chopped apples to a medium sauce pot with the water & sugar of your choice
Cook the apples over medium to high heat with the lid on for about 10 minutes to rapidly boiling, Take off the lid and reduce the heat to a simmer until the apples are translucent & mushy.
Remove from heat & transfer the apples to a blender with about ½ cup of the plant milk & the cornstarch ~ puree smooth
Transfer the apple puree to the same sauce pot that you cooked the apples & add the remaining plant milk.
Return to the stove over medium to high heat whisking constantly until it comes to a boil.
Add the vanilla extract then strain through a mesh strainer onto a sheet pan (no parchment paper or grease!)
Refrigerate the custard until it is completely cold & set
Meanwhile whip the vegan butter & salt with an electric mixer with the whip attachment until it is aerated & light & fluffy and starting to look whiter in color (about 5 minutes) scraping the bottom & sides of the bowl often
Add the sifted confectioners sugar & whip for another minute or two until smooth scraping the bottom & sides of the bowl often
Add the cold custard to the vegan butter while continuing to whip on high speed until all the custard is incorporated, scraping the bottom & sides of the bowl often
Continue whipping until the buttercream is smooth & silky