Arrange baking racks to the middle of the oven and preheat to 350 degrees.
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 fitted with the paddle attachment 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 and add the flour mixture. Mix until the dough comes together in a mass with no flour streaks, about 1 minute. Stir in 3/4 cup of the chocolate chunks.
Divide the dough in half. Shape each half on a lightly floured counter into a firm log about 12 inches long and 2 inches wide - dampen your hands if the dough feels sticky. Arrange the logs on the baking sheet about 3 inches apart.
Bake 30-40 minutes, until the logs are firm to the touch and there are a few cracks on the surface. 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 300 degrees. Bake 5 minutes, then gently turn the biscotti unto the other cut side and bake for another 5 minutes.
Transfer the biscotti to a rack to cool.
Melt the remaining chocolate in a heatproof bowl set over simmering water, or in 30-second increments on high in a microwave, stirring each time.
Drizzle the melted chocolate over the cool biscotti.
To drizzle the biscotti with the melted chocolate without making a mess, set them on a cooling rack over a piece of parchment paper, either on their sides or upright. Dip a small spoon or fork into the chocolate and drizzle away. Let the chocolate cool and set before packing in bags or boxes.
Keep the biscotti in a covered container for up to a week.
Note on measuring flour: If you don't have a scale, be sure to measure your flour properly by fluffing it up first, then spooning into your measuring cup and leveling with a butter knife. If you scoop the flour directly from the canister, it will compact it and produce a dry dough.
Note on eggs: The amount of eggs needed can vary depending on the size and how dry your flour is — I've found that even eggs marked "large" can sometimes be smaller than expected. If you find your dough looks a bit dry, lightly beat an additional egg and dribble it into your mixing bowl until the dough is soft and slightly sticky.