Line a large rimmed baking sheet with parchment paper.
Whisk together flour, cocoa, baking soda and salt in a bowl.
Beat the butter in an electric mixer until fluffy; add the sugar and mix at high speed until combined, about 30 seconds. Scrape down the bowl if needed, then beat in the eggs. Lower the mixer speed to stir in the flour mixture and mix until just combined; stir in the chocolate chunks.
Divide the dough in half. Shape each half on a lightly floured counter into a log about 12 inches long and 3 inches wide. Arrange the logs on the baking sheet about 3 inches apart.
Bake 30 - 40 minutes, until the logs are firm to the touch. Cool on a rack 10 minutes.
With a large, sharp knife, slice each log into ¾-inch wide slices on a slight diagonal and put them cut side down on the baking sheet.
Lower oven to 325 degrees. Bake 5 minutes, then gently turn the biscotti unto the other cut side and bake for another 5 minutes.
The biscotti are great plain or dip them into some chopped, melted chocolate if you like.