Place the chocolate chips, coffee and water in a microwave safe bowl. Microwave until chocolate is almost melted. Remove and whisk until mixture is smooth. Set aside to cool.
Using a hand held mixer, beat butter and margarine together in a large bowl until creamy. Gradually beat in sugar until light and fluffy.
Gradually beat in egg yolks one at a time, beating well after each one. Slowly add the flour and beat at a low speed. Add the cooled chocolate to the mix and beat again.
In a separate bowl, beat egg whites until foamy. Then increase speed and beat until stiff.
Fold egg white mixture, one third at a time, into the chocolate mixture until blended thoroughly.
Grease a 23cm springform tin or a cake tin with deep sides. Wrap the bottom of the tin securely with greaseproof paper or foil, so it comes up about 1.5cm on the sides.
Pour the mixture into the cake tin. Bake for I hour. The cake is ready when a skewer inserted about 1.5cm from the side of the tin comes out clean. Note that the cake is very dense, so the top may crack when baking.
Cool in the tin for 10 minutes to allow the cake to set. Then remove from the tin, and allow to cool completely, which may take several hours.
Sift icing sugar lightly over the top for decoration.