Add the powdered sugar, cocoa powder, vegan butter, vanilla and around half the soy milk to the bowl of your stand mixer.
Start mixing on low, gradually moving to a higher setting until thick, smooth and creamy. Add the extra soy milk in slowly, only as much as needed (see notes).
It should be thick, but still be easily spreadable so it won't go sliding off a cake. So if it's not thick enough, add a little more powdered sugar.
