Preheat the oven to 350°F. Lightly grease (or line with parchment) one large (about 18" x 13") baking sheet., In a medium-sized bowl, beat the butter, sugar, salt, spices, vanilla, and baking powder until the mixture is smooth and creamy., Beat in the eggs; the batter may look slightly curdled., At low speed of your mixer, add the flour and crystallized ginger, stirring until smooth; the dough will be quite soft and sticky, but should hold its shape when you drop it from a spoon., Transfer the dough to the prepared baking sheet, and shape it into a rough log about 14"-long. It will be about 2 1/2" wide, and about 3/4" thick. Using your wet fingers, smooth the top and sides of the log., Bake the dough for 25 minutes., Remove it from the oven, and allow it to cool on the pan anywhere from 5 to 25 minutes; just work it into the schedule of whatever else you're doing in the kitchen., Using a spray bottle filled with room-temperature water, lightly but thoroughly spritz the log, making sure to cover the sides as well as the top. Softening the crust just this little bit will make slicing the biscotti much easier., Reduce the oven temperature to 325°F., Wait another 5 minutes, then use a serrated knife to cut the log into 1/2" to 3/4" slices. Cut at a 45° angle, for long biscotti; cut crosswise slices, for shorter 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., Set the biscotti on edge on the prepared baking sheet., Return the biscotti to the oven, and bake them for 25 to 30 minutes, until they feel very dry. 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., Remove the biscotti from the oven, and transfer them to a rack to cool., Once the biscotti are cool, store airtight, to preserve their texture., If biscotti aren't as crunchy as you'd like (and the weather is dry), store them uncovered, overnight, to continue drying.