With a hand mixer (or stand mixer with a whisk attachment), beat the butter on medium speed for about 2 minutes, until creamy.
Sift in the cocoa powder if clumpy, and add the powdered sugar, vanilla and soy milk. Beat on low speed for 30 seconds, then increase the speed to high and beat for 2-3 minutes, until combined and fluffy.
To thin the consistency (if needed) add more milk, or to thicken add more powdered sugar.