Mix melted butter and cocoa powder together until smooth.
Beat in sugars. Beat in eggs, one at a time, mixing with each addition. Add vanilla.
Add salt, baking soda and flour.
Stir in chocolate chips.
Cover dough and refrigerate for 30 minutes.
Line a baking sheet with parchment paper.
Sprinkle the work surface with a couple of tablespoons of flour.Transfer dough onto the work surface then sprinkle dough with a small amount of flour.Work the dough into a ball then cut the dough in half.Note: the dough will be wet and sticky---this is normal. Just add enough flour to keep it from sticking to your hands and work surface too much. I like kneading the dough a bit with the flour to cover the outside while I shape it. Form each dough portion into a 9" log.
Place biscotti logs onto a parchment paper-lined baking sheet. They will spread when baked so keep 3-4" in between each log or bake on separate sheets.
Flatten the logs a bit with your hands. They should be about 9" long and 3" wide.
Bake at 350 degrees for 25 minutes. Allow cookie logs to rest 5 minutes.Slice logs into 3/4"-1" sections. Turn each section onto its flat side. Arrange biscotti slices on baking sheet.
Bake biscotti for an additional 9 minutes. Flip biscotti over for even baking then return to oven and bake another 9 minutes.
Let cookies cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Dip in chocolate, if desired.