Preheat oven to 180 C / Gas 4. Grease a 20x30cm tin. 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 bicarbonate of soda. Pour in milk and eggs. Mix the cooled chocolate mixture into the batter. Pour into prepared tin.
Bake at 180 C / Gas 4 for 30 to 35 minutes or until a skewer inserted into the cake comes out clean. Spread with buttercream icing.