1) Preheat the oven to 350°F. Lightly grease the biscotti pan
2) In a medium-sized bowl, beat the butter, sugar, salt, vanilla, and baking powder until the mixture is smooth and creamy. Beat in the eggs; the batter may look slightly curdled. Add the flour using the low speed of your mixer, stirring until smooth; the dough will be sticky.
3) Scoop the dough into the prepared biscotti pan, shaping it into a rough log and leaving about 3/4" bare around the edges of the pan. The dough should be mounded in the center.
4) Bake the dough for 25 to 30 minutes, until it's golden brown all over. Remove it from the oven, and allow it to cool in the pan for 30 minutes. Remove from the pan. 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. Wait 5 minutes before slicing. Softening the crust just this little bit will make slicing the biscotti much easier. Use a serrated knife to cut diagonal 1/2" to 3/4" slices.
5) Reduce the oven temperature to 325°F.
6) Set the biscotti on edge on a large baking sheet; there's no need to grease the pan. Return the biscotti to the oven, and bake them for 25 to 30 minutes, till they feel very dry and are beginning to turn golden. They'll still feel a tiny bit moist in the center, if you break one in half; but they'll continue to dry out as they cool. Remove the biscotti from the oven, and transfer them to a rack to cool.