In the bowl of a stand mixer, combine the room temperature butter, powdered sugar, and salt.  Using the paddle attachment start to cream on low for 2 minutes, then gradually increase the speed to medium high and beat until light and fluffy.

Add the egg yolk and beat for another few seconds until the yolk has almost disappeared.

Add the flour, working in two batches, and mix on low very carefully until everything is just combined.  Finish the dough by hand with a spatula so as to not over work the dough.  It should still be crumbly.

Pour the dough out onto a big piece of plastic wrap and form a round, tightly wrapped disc.  Chill in the refrigerator at least an hour or freeze up to a couple months. (Double wrap the dough if freezing.)

When you're ready to roll and cut the cookies, remove the dough from the fridge or freezer.  If frozen, place in the refrigerator overnight, then let soften a bit on the counter.

When the dough isn't rock solid, roll half the disc out between two pieces of parchment paper and cut the cookies out using your shape of choice.   The dough can be re-rolled 2 times till there's hardly any left.  Repeat with the other half of the disc.

Place the cut out pieces of dough on parchment paper in the refrigerator to chill for a few minutes before baking.

When ready to bake, bake in a preheated 350F/ 180C degree oven for about 10 minutes until the edges are just starting to turn golden.  (Larger cookies may need a few minutes more to bake.) Let cool on the cookie sheet, then move to a cooling rack.

If you want the ghosts extra white, dust the top cookies with a little confectioner's sugar before sandwiching with the ganache.

Heat the semi-sweet chocolate over a double boiler or in the microwave in intervals until starting to soften.  Stir with a spatula until it is all melted together.  Be careful not to burn the chocolate.

Next, stir together the Nutella, melted semisweet chocolate, and the sea salt.  Set aside until cool.

Once the ganache is cooled and set a bit, take one shortbread cookie, bottom side up, and place about a tablespoon of ganache in the center and spread out to the edges with an offset spatula or knife.  Use more ganache if you have bigger cookies.

Place the other cookie bottom side down so it is touching the ganache on top of the first cookie to make a sandwich.  Enjoy!