Add the butter, salt and vanilla to a large mixing bowl. Mix until smooth..
Add both cocoa powders and mix on low until combined, then on medium until well incorporated. Scrape down the sides of the bowl and mix again.
Add in 3 cups of confectioner's sugar. Mix on low until combined, then on medium until fully incorporated.
Add 2 tablespoons of milk plus the last 2 cups of powdered sugar. Mix on low until combined, then on medium until fully incorporated. Scrape down the sides of the bowl, then mix again.
Add vegan whipping cream only a tablespoon at a time, and mix after each addition.