For the Dough:

1. In a large bowl, fitted with a paddle attachment, cream together the butter, maple sugar and granulated sugar until it is well combined.  Stop to scrape down the sides, occasionally.  At this point, the mixture will seem quite dry.    Add the maple syrup and vanilla and maple drops (if using), and continue to mix on low.  The addition of the wet ingredients will help the mixture to come together into a smooth and creamy mixture. 
2.  Add the egg and  mix until well blended, before adding the egg white.  Continue mixing until the butter mixture becomes light and creamy.
3. In a separate bowl, whisk together the flour, baking powder and salt.  Gradually stir the flour mixture into the butter mixture and combine thoroughly on med-low speed.
4. Combine the nuts, coconut and caramel bits into the dough and continue to mix just until the dough forms a moist sticky mixture and all ingredients are well incorporated. 
5. Scoop the dough onto a large piece of plastic wrap and with dampened fingers, form a log about 12 inches long, 2 1/4 inches wide and 3/4 inches thick.  It does not have to be exact, these are rough dimensions.  Wetting your fingers with a little water will help to keep the dough from sticking to them. 
6.  Wrap the dough and chill for at least 30-45 minutes or overnight.
7. If you are planning to bake immediately after chilling, preheat the oven to 325 degrees with the rack placed in the center of the oven.  
8. Line a large baking sheet with parchment paper or a silpat. 
9. Place the reserved egg yolk, in a small bowl or jar and add 1 teaspoon of water.  Whisk to combine.

To Bake Biscotti:

1. Preheat oven to 325 degrees F. and line a baking sheet with parchment paper or a silpat.
2. Remove the plastic wrap, and place the log of dough on the prepared baking sheet.  
3. Using a pastry brush, paint the log with the egg wash covering the top and sides.  If desired, sprinkle the top of the biscotti log with sparkling sugar. 
4. Place the baking sheet in the oven and bake until the log is golden brown, firm to touch and a tester inserted into the center comes out clean, about 28-35 minutes.  The internal temperature taken with a digital thermometer should be 195 degrees F. 
5. Remove the baking sheet from the oven and let cool on the pan for about 15 minutes.  Gently transfer to a cookie rack and let cool completely at least 30 minutes.   But if time allows, let the cookie log cool for 2- 3 hours.   This will keep the caramel from smearing when cut.  There is no need to cover the baked log.  Simply allow the log to cool uncovered on a cooling rack until ready to cut.
6. Reduce the oven temperature to 250 degrees for the second bake.
7. Transfer the biscotti log to a cutting board.  Using a serrated knife, cut the biscotti crosswise into slices about 1 cm (1/4 inch) in thickness.  This will net you between 25-27 cut cookies, including the end pieces.  (I use a gentle sawing motion to break through the crust.  After that, a firm push down on the knife is all that is needed to cut the biscotti.)
8. Return the slices to the baking sheet, arranging them cut side down.  Don’t worry if the biscotti touch, as they will not spread when baked.
9. Bake the biscotti for about 14 minutes.  Remove baking sheet from the oven and allow to cool 5 minutes. 
10. Turn each cookie over and bake an additional 14 minutes, just until the edges turn golden in color.
11. Transfer the biscotti to a cooling rack and allow to cool completely.  The biscotti will continue to crisp as they cool. 
12. Store the cooled biscotti in an air tight container.  They will keep for up to a month.  For longer storage, the biscotti may be frozen.