To prepare frosting: Beat vegan butter in a stand mixer fitted with a whisk attachment on medium speed until smooth, about 1 minute. With the mixer running on low speed, gradually add confectioners' sugar and cocoa, stopping to scrape down the sides as necessary. Increase speed to medium-high and add water and vanilla, beating until smooth, about 2 minutes. Spread the frosting on top of the cooled cake. Chill the cake for 1 hour before slicing.