Beat cream cheese until fluffy. Gradually beat in sugar. Add egg. Blend until smooth. Stir in flour, sugar, cocoa, baking soda and salt. Beat together water, oil, vinegar and vanilla. Gradually add flour to flour mixture, blending well. Turn into a greased 23 cm square baking pan. Drop cream cheese mixture by spoonfuls over batter. Sprinkle with chocolate pieces and walnuts. Bake at 180ºC for 30-50 minutes until cake tester comes out clean and top springs back when touched. Cool on wire rack. To serve, cut into squares from pan.