Microwave 1 cup chocolate chips in a small, microwave-safe bowl on high for 1 minute; stir. Cook at additional 10 to 20 second intervals, stirring until smooth. Cool to room temperature.
Beat melted chocolate, sour cream, oil and eggs in large mixer bowl. Beat in sugar, flour, bicarb soda and salt. Dissolve coffee powder in water; gradually beat into batter. Pour into greased 20x30cm baking pan. Sprinkle the remaining 1 cup chocolate chips and the chopped nuts over the top of the cake.
Bake in a preheated 180 degrees C for 30 to 40 minutes or until cake tests done.