1) Preheat the oven to 350°F. Lightly grease (or line with parchment) one large (about 18" x 13") baking sheet.
2) In a medium-sized bowl, beat the butter, sugar, salt, vanilla, and baking powder until the mixture is smooth and creamy.
3) Beat in the eggs; the batter may look slightly curdled.
4) At low speed of your mixer, add the flour, stirring until smooth; the dough will be sticky.
5) Divide the dough in half, leaving half in the bowl, and placing half on the prepared pan. If you have a scale, half the dough is about 10 1/4 ounces (290g). Volume-wise, half the dough is a generous 1 cup.
6) Shape the dough on the pan into a log that's about 14" long x 2 ½" wide. Straighten the log, and smooth its top and sides; a wet spatula or wet bowl scraper (or wet fingers) works well here. Place the pan in the freezer while you make the chocolate dough.
7) To prepare the chocolate dough, add the cocoa powder and espresso to the vanilla dough in the bowl, stirring to combine. Stir in the chocolate chips.
8) Remove the pan from the freezer. Using your wet fingers, spread the chocolate dough atop the vanilla dough, pressing it down the sides to entirely enclose the vanilla dough, if desired. You can also just leave the sides bare. Sprinkle the coarse sugar atop the dough, pressing it into the surface gently with your fingers.
9) Bake the dough for 25 minutes. Remove it from the oven, and allow it to cool on the pan anywhere from 10 to 25 minutes; just work it into the schedule of whatever else you're doing in the kitchen. While the biscotti are cooling slightly, reduce the oven temperature to 325°F.
10) If you've used parchment on your baking sheet, use it to lift the biscotti off the sheet onto a flat surface. If you haven't used parchment, carefully lift the biscotti off the sheet onto a flat surface. Using a serrated knife or sharp chef's knife, cut the biscotti crosswise into 3/4" slices. Or cut the biscotti on the diagonal — for fewer, longer biscotti. As you're slicing, be sure to cut straight up and down, perpendicular to the pan; if you cut unevenly, biscotti may be thicker at the top than the bottom, and they'll topple over during their second bake.
11) Set the biscotti, on edge, back on the baking sheet. Return the biscotti to the oven, and bake them for 30 to 40 minutes, till they feel very dry and are beginning to turn golden. They'll still feel a tiny bit moist in the very center, if you break off a piece; but they'll continue to dry out as they cool.
12) Remove the biscotti from the oven, and transfer them to a rack to cool.
13) If you haven't topped the biscotti with coarse sugar, glaze them once they're cool. Mix the confectioners' sugar and water, and drizzle it atop the biscotti.
Yield: 21 biscotti, about 4" to 5" long.