Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch pan. In a microwave-safe bowl, microwave chocolate, margarine and water until melted. Stir occasionally until chocolate is smooth. Allow to cool to room temperature.
In a large bowl, mix together flour, sugar, salt and soda. Pour in milk and eggs. Mix the cooled chocolate mixture into the batter. Pour into prepared pan.
Bake at 350 degrees F (175 degrees C) for 30 to 35 minutes, or until a toothpick inserted into the cake comes out clean. Frost with Best Buttercream Frosting.