Preheat oven to 350 deg F. and butter and flour a large baking sheet. In a bowl, whisk together flour, cocoa powder, baking soda, and salt. In another bowl with an electric mixer beat together butter and granulated sugar until light and fluffy. Add eggs and beat until combined well. Stir in flour mixture to form a stiff dough. Stir in walnuts and chocolate chips. On prepared baking sheet with floured hands, form dough into two slightly flattened logs, each 30 cm long and 5 cm wide, and sprinkle with confectioners' sugar. Bake logs 35 minutes, or until slightly firm to the touch. Cool biscotti on baking sheet 5 minutes. On a cutting board, cut biscotti diagonally into 2 cm slices. Arrange biscotti, cut sides down, on baking sheet and bake until crisp, about 10 minutes. Cool biscotti on a rack. Biscotti keep in air-tight containers 1 week and, frozen, 1 month.