Preheat oven to 190 C / Gas 5. Lightly grease two 12 inch pizza pans.
Stir together flour, bicarbonate of soda and salt and set aside.
Using a large bowl, cream together butter, cream cheese and sugars. Add eggs and beat until fluffy. Add flour mixture to creamed mixture, blending until dough becomes stiff. Stir in chocolate chunks and walnuts until well blended.
Divide dough in two. Press each half into the prepared pans. Bake 20-25 minutes or until lightly browned around the edges. centres should be firm. Cool in the pans on wire rack. Serve by cutting into wedges.