Sift the cocoa into the mixing bowl. With and electric mixer on medium-high speed, slowly add the melted butter to the cocoa in the bowl. With the mixer still running add the sweetened condensed milk. Scrape down the sides of the bowl as needed. Add the vanilla and beat until combined and the frosting is thick. Spread the frosting on cooled cake or cupcakes. Let stand for 2 hours for the frosting to set.