1. Combine the chocolate and margarine in a bowl, melt in microwave (30 seconds at high, then stir and repeat until melted). Stir in the soy milk and vanilla. Add the sugar gradually, beating with a fork. Continue beating, until the mixture is smooth; it will be runny.
2. Refrigerate the mixture for 1 hour, beating it every 15 minutes (the icing will stiffen).