Adjust an oven rack to the middle of the oven. Preheat the oven to 350F [180C]. Line a baking sheet with parchment paper.
In a stand mixer fitted with a paddle attachment, mix together the flour, cocoa powder, baking soda, and salt on low speed, until just combined, about 15 seconds. Add the eggs, sugar, and vanilla, and turn the speed up to medium, and mix until it just comes together into a thick but wet dough, for about 1 minute. Mix the chocolate, nuts, dried cherries, and candied citrus until just combined. You may need to stir with a rubber spatula to make sure they are evenly distributed.
Form the dough into two equal logs 8 in [20 cm] wide by 3 in [7.5 cm] long and place them on the prepared pan. Leave about 3 in [7.5 cm] between them, so they have room to spread.
Bake for 30 to 35 minutes, until the dough is set but still soft.
Allow the logs to cool, just until they are cool enough to handle, 10 to 15 minutes. With a sharp serrated knife, cut the logs into slices 1/2 in [1.3 cm] thick. Keep the oven on.
Lay the slices on the baking sheet and bake again for 10 minutes. Flip the biscotti over and bake for about 10 more minutes, or until crisp.
Cool on the baking sheet and store in an airtight container. These cookies last for weeks. Before serving, drizzle with melted chocolate (see note) and sprinkle with a few flakes of salt, if desired. Let the chocolate set before serving.