Preheat the oven to 350°F. Lightly grease a biscotti pan, or line a baking sheet with parchment., In a medium bowl, beat together the butter, sugar, salt, vanilla, and baking powder until the mixture is smooth and creamy. Beat in the eggs one at a time, scraping the bowl after each addition. Add the flour and mix at low speed until the mixture is smooth; the dough will be sticky., Scoop half the dough onto one side of the prepared pan and, using wet hands, pat it into a 10” x 2” rectangle; it should be about 1” thick. Repeat with the other half of the dough., Bake for 20 to 30 minutes, until golden brown all over. Remove from the oven and let cool for 25 to 30 minutes. Reduce the oven temperature to 325°F., Carefully transfer the logs to a cutting board and use a spray bottle filled with room-temperature water to lightly spritz them. Wait 5 minutes before using a serrated knife to slice biscotti 1/2” to 3/4” wide. Take care to draw the knife back and forth through the baked log instead of pressing down. Slicing this way will prevent the biscotti from crumbling., Return the slices to the baking sheet, standing them up with 1/2” between them, so air can circulate around them. Bake for another 25 to 30 minutes, until dry and golden. Remove from the oven and transfer to a rack to cool., Dip the biscotti in melted white chocolate, if desired., Store airtight at room temperature; they'll stay good for weeks.