Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan.
In a large bowl, stir together the flour, sugar, cocoa, bicarb soda and salt. Combine the hot water and instant coffee then add to the dry ingredients along with the oil, vinegar, vanilla and eggs. Mix until smooth and well blended. Spread batter evenly into the prepared pan. Sprinkle with the 1/4 cup of sugar.
Bake for 45 to 60 minutes in the preheated oven, until a toothpick inserted comes out clean.
Make the topping while the cake is cooling. In a medium-size mixing bowl beat together the cream cheese, sugar, vanilla and egg until smooth. Stir in the chocolate chips and walnuts. Spread over cooled cake.