Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
Combine the flour, cocoa powder, baking soda, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter, sugar, and brown sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla.
Reduce the mixer speed to low. Gradually add the flour mixture, mixing just until combined. Stir in the chocolate chips.
Transfer the dough to the prepared pan. Shape the dough into a rectangle about 3 & 1/2" wide and 3/4" tall. The dough will be a bit sticky, so dust your fingertips with a bit of flour if necessary to help you shape it.
Bake for 25 minutes. Remove the pan from the oven and allow to cool for 10 minutes (or until is cool enough to handle).
Use the parchment paper to transfer the biscotti to a cutting board. Cut the biscotti into 3/4" crosswise slices.
Line the baking sheet with a fresh sheet of parchment paper or clean silicone liner. Place the biscotti, cut side down, onto the pan.
Bake 18 to 22 minutes, or until browned. Cool on the pan for 10 minutes. Then transfer the cookies to a wire rack to cool completely.
Notes
*Dark cocoa powder gives these biscotti their dark color and a richer chocolate flavor. You can substitute regular cocoa powder, but you'll get a lighter color and a little less chocolate flavor.