2 cups flour
1/2 cup unsweetened cocoa powder
1 teaspoon baking soda
1 teaspoon salt
3/4 stick (6 tablespoons) butter, remove from refrigerator 30 minutes before use
1 cup sugar
2 large eggs
1 cup chopped walnuts
1 cup semisweet chocolate chips
Powdered sugar, as needed
Whisk together flour, cocoa, baking soda and salt in large bowl; set aside.
Beat together butter and sugar gradually with electric mixer until light and fluffy, about 2 minutes. 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 inches long and 2 inches wide. Place logs on baking sheet. Sprinkle each with powdered sugar. Bake in 350° F oven for 35 minutes or until tops are slightly firm to touch. Cool biscotti on sheets for 5 minutes.
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 inch wide slices. Place biscotti, cut sides down, on baking sheet and return to oven and bake until crisp, about 10 minutes. Cool on wire rack.