1. Whisk together flour, cocoa, baking soda and salt in large bowl; set aside.
2. Beat together butter and sugar gradually with electric mixer until light and fluffy, about 2 min. Add eggs and beat until well blended. Add flour mixture to form a stiff dough. Stir in walnuts and chocolate chips. Grease and flour a large baking sheet. With floured hands, form dough into two flattened logs, about 12" long and 2" wide. Place logs on baking sheet. Sprinkle each with powdered sugar. Bake in 350°F oven for 35 min. or until tops are slightly firm to touch. Cool biscotti on sheets for 5 min.
3. Carefully transfer biscotti with wide spatula to wooded cutting board. With a very sharp serrated knife or an electric knife, slice biscotti diagonally into 3/4" wide slices. Place biscotti, cut sides down, on baking sheet and return to oven and bake until crisp, about 10 min. Cool on wire rack.