Preheat the oven to 350°F. Lightly grease (or line with parchment) one large (about 18" x 13") baking sheet., Combine the flours and coconut, stirring until well blended. Set aside., In a medium-sized bowl, beat together the butter, sugar, salt, vanilla, coconut extract or flavor, and baking powder until the mixture is smooth and creamy., Beat in the eggs until well incorporated. The batter will slightly curdled, but it will smooth out when you add the flours., With your mixer running at low speed, add the flours and coconut, stirring until blended and the dough is sticky., Plop the dough onto the prepared baking sheet. Divide it in half, and shape it into two 9 1/2" x 2" logs, about 3/4" tall. Straighten the logs, and smooth their tops and sides; a wet spatula or wet bowl scraper works well here. Sprinkle with coarse white sparkling sugar, pressing it in gently., Bake the dough for 25 minutes, then remove it from the oven., Using a spray bottle filled with room-temperature water, lightly but thoroughly spritz the logs, 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 5 minutes, then use a sharp chef's knife or serrated knife to cut the log crosswise into 1/2" to 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., 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 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., Remove the biscotti from the oven, and transfer them to a rack to cool. Store airtight at room temperature; they'll stay good for weeks.