Step 1

Process the pretzels in a food processor until finely ground and the consistency of flour. Measure 1/2 cup of the ground pretzels into a bowl (discard any remaining ground pretzels). Whisk in the flour, cornstarch, confectioner' sugar and fine salt.

Step 2

Add the butter and brown sugar to a stand mixer fitted with the paddle attachment and beat on medium speed, scraping down the sides of the bowl as necessary, until light and fluffy, about 2 minutes. Add half of the flour mixture and mix on low just to combine, about 20 seconds. Add the remaining flour mixture and mix on low until it comes together to form a dough.

Step 3

Turn the dough out onto a 14-inch piece of parchment and form into a ball. Place the parchment in a 9-by-13-inch jelly roll pan or baking sheet and press the dough into an even rectangle the size of the pan using the bottom of a measuring cup. Cut the dough in half horizontally, then cut in half vertically to make 4 quarters. Cut each quarter into 10 slices to make 40 narrow rectangles (the ends might be less than perfect, but that’s ok). Freeze for 20 minutes.

Step 4

Position a rack in the middle of the oven and preheat to 350 degrees F.

Step 5

Add the baking soda to 1/2 cup warm water and mix well.

Step 6

Brush the dough evenly with the baking soda mixture and then with the beaten egg. Sprinkle with the pretzel salt.

Step 7

Bake, rotating the pan halfway through, until the shortbread is deep brown and cooked through, 45 to 55 minutes. Let stand for 5 minutes in the pan, then carefully lift the shortbread onto a cutting board. Using the previous cuts as a guide, carefully cut into bars. Let cool completely, about 25 minutes.

Step 8

Melt the chocolate chips and coconut oil in a double boiler over low heat. Transfer to a small bowl. Dip the lower third of each cookie in the chocolate and place on a clean piece of parchment. Refrigerate for 10 minutes to allow the chocolate to harden. Store the cookies in an airtight container for up to 5 days.