Marinate the duck:

1. Trim the duck of any excess fat. 
2. Score the duck by gently cutting on the diagonal through the skin and partially through the fat in even ¼ inch strokes (do not cut into the meat itself.) 
3. Turn the duck 45 degrees and score again, so that you have a diamond or cross hatch pattern.
4. Combine the plum wine, hoisin sauce, soy sauce, and sugar and stir until the sugar is dissolved. 
5. Pour the sauce over the duck. Submerge the duck making sure that the breasts are well coated.
6. Cover and refrigerate overnight or a minimum of 4 hours.

Cook the Duck:

1. Heat a large 12″ pan over medium heat for several minutes. Dry the duck on a paper towel and lay it skin side down and cook for 3 minutes. 
2. Flip the duck and cook the other side for another 3 minutes. 
3. Cover the pan with a lid, lower the heat to medium low, and cook for 2 minutes.  
4. Take the pan off of the heat and let the duck sit, with the lid still covering it, while you make the sauce.

For the Sauce:

1. Pour the marinade into a small saucepan and cook over medium high heat for 6-8 minutes until it is thickened and reduced (you should have about 3/4 cup of sauce).
2. Add the umeboshi plums and carefully blend with an immersion blender. (You can also do this in a standard blender with the pour center removed. Cover the opening with a kitchen towel and carefully blend on low.)
3. Add the butter and swirl it around to melt it. Transfer the sauce to a small bowl. 

Slice and Serve:

1. Cut the duck on an angle into thick ½ inch slices.
2. Transfer to a platter and serve immediately with the sauce on the side.