Preheat oven to 350°F.
In a large bowl, beat butter, sugar, eggs and vanilla until smooth. In a medium bowl, stir together the flour, cocoa, baking powder and salt. Add to the egg mixture and stir until almost combined; add the chocolate chips and walnuts and stir just until blended.
On a cookie sheet that has been sprayed with nonstick spray, shape the dough into a 10” long log. Flatten it with your hands until it’s 3”-4” wide.
Bake for 30–35 minutes, until firm and cracked on top. Transfer the log to a wire rack to cool for a bit and reduce the oven temperature to 275°F.
When the log is cool enough to handle (they tend to crumble when they’re hot), transfer to a cutting board and cut diagonally into 1/2”–3/4” slices with a serrated knife. Place the slices upright on the cookie sheet, spacing them about 1/2-inch apart so that there’s room for the air to circulate between them, and return to the oven for 30 minutes. If you like, turn the heat off and leave the biscotti inside the oven until it cools down to make them even harder.
Makes about 15 biscotti.