Preheat the oven to 425°F.

Pour the condensed milk into an oven proof dish.  Cover it tightly with foil.

Place it in a large dish or pan and fill the bottom pan with hot water at least half way up the dulce de leche dish.

Bake it for 1 1/2 hours.  Remove the foil (be careful of the steam - take the foil off pointing away from you.)  Check the color and the thickness.  If it isn't thick enough, recover, add more water and bake for another 15 minutes.  Check it, repeat if necessary.

Cool completely.  If it is at all lumpy when you stir it, smooth it out by whisking. 

Preheat the oven to 350°F.  Place the pecans on a baking tray and toast for 5 to 9 minutes depending upon if  the pecans are cut or whole.  They will be come fragrant and pick up a little color.

Line several baking sheets with parchment paper and set aside.

Combine the flour and pecans in the bowl of a processor.  Process  until the nuts are very finely cut in.  

Add the powdered sugar and salt and pulse several times to mix.

Beat the butter and vanilla in a mixing bowl until creamy.  Add the flour/pecan mixture and mix until it comes together. 

Divide the dough in half (about 255 grams or about 9 ounces each).  Roll each piece into a 12" log.  This will make baked cookies about 1 1/2 inches.  If you want larger cookies, roll into a 10" log.  The cookies don't spread much so they are a little bigger than unbaked. 

Wrap in plastic wrap refrigerate to firm up or freeze for longer storage.

If using sanding sugar to finish the tops or sanding sugar with salt mix the sea salt to taste with the sugar.  Set aside.

Cut the cookies about 1/3" thick.  At the bakery, this was easiest to do by putting our fingernail where we wanted to cut, placing the knife next to it and slicing straight down.  Place on parchment paper lined baking sheets.  Sprinkle liberally with the sanding sugar or sanding sugar with salt.

Bake for 10 to 12 minutes until the edges begin to color.  

Combine the chocolate and shortening or cocoa butter and melt it over hot water or microwave at half power until melted.  Stir well.  Do not whisk as it is too easy to incorporate air bubbles, which you definitely do not want.  

Place the glaze in a narrow container as it will be easier to dip the cookies.   Dip the top of each cookie and set on parchment or waxed paper.  Immediately, top the cookie with a pecan half or the sugar/pecan or sugar/pecan/salt mixture.  

Allow to set for several  hours or refrigerate briefly to set.

Pair the cookies for size and shape.

Turn the bottom cookie upside down with its mate above it right side up.  Fit a piping bag with a #3 plain tip.  Pipe a mound of dulce de leche  on the bottom cookie.  Immediately place the top cookie on, pressing down lightly to move the filling to the edges.

After the glaze is set, the cookies can be stored at room temperature.  If you are in a hurry, refrigerate them briefly.  This may dull the glaze somewhat.  They can be stored at room temperature for several days.