1. Heat oven to 375 degrees F.
2. Sift together the flour, cocoa powder, baking soda and salt into a medium-size bowl.
3. In a large bowl, beat the butter with an electric mixer on medium-high speed until creamy, about 1 minute. Add the granulated sugar and brown sugar; beat until the mixture is fluffy, about 2 minutes. Beat in the eggs, one at a time, beating well after each addition. Beat in the vanilla until well blended.
4. On low speed, beat the flour mixture into the butter mixture until well blended. Stir in the chocolate pieces and walnuts.
5. For each cookie, drop a heaping tablespoonful of dough onto ungreased baking sheet, spacing the mounds about 2 inches apart.
6. Bake the cookies in 375 degree F oven until lightly golden, 9 to 11 minutes. Let the cookies cool on the baking sheet on a wire rack for 2 minutes. Transfer the cookies to the wire rack; let cool.
7. Serve cookies warm or at room temperature. Cookies can be stored in an airtight container at room temperature for up to 5 days. Makes 36 cookies.