1) Place the racks in the center and upper third of the oven, and preheat the oven to 325°F. Line two baking sheets with parchment paper, or grease them lightly.
2) In a medium bowl, whisk together flour, cocoa, baking powder, and salt.
3) In the bowl of an electric stand mixer fitted with a paddle attachment, beat the butter and sugar on medium speed until light and fluffy, about 3 to 5 minutes.
4) Reduce the mixer speed to low, and add the egg and egg yolk. Beat until well incorporated, scraping down the bowl as needed.
5) Beat in the rum or vanilla extract and orange zest.
6) Add the flour mixture all at once to the butter mixture, and beat on low speed until well combined. The dough may seem a bit dry, but that's OK. Mix in the dried fruit.
7) Divide the dough between the two prepared baking sheets. If the dough is shaggy, that's fine, just press it together.
8) Shape the dough into logs about 1 1/2" wide and 8" long.
9) Beat the reserved egg white, and brush over the logs. Sprinkle generously with granulated sugar, if desired.
10) Bake the biscotti for 20 minutes. Swap racks, and bake for another 15 to 20 minutes, until cooked through.
11) Remove the biscotti from the oven, and let them rest until cool enough to handle.
12) Use a serrated knife to gently slice biscotti into 1"-thick slices.
13) Allow to cool completely before storing. The biscotti will stay fresh, in an airtight container at room temperature, for about 1 week.
Yield: 2 dozen biscotti.