Using a food processor (I use my mini Kitchen Aid food chopper), blitz the pretzels into a fine powder. You don't want any big chunks! You should have 160 grams or 1 1/2 cups pretzel flour.
In a medium bowl, combine the pretzel flour, all-purpose flour, baking powder and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, add the butter and sugar. Mix on medium speed until light and fluffy, about 4 minutes. Scrape down the sides of the bowl and add in the egg. Mix again until combined and then add in the reserved pretzel flour, all-purpose flour, baking powder and salt. Mix until combined. Remove dough from bowl and place onto a sheet of parchment paper. If there are any dry spots still stuck in the bowl, feel free to incorporate them into the dough by kneading it together.
Pat dough down into a flat disc and split in two mounds, this just makes it easier to roll out.
Roll out each mound between two sheets clean sheet of parchment or with a lightly floured rolling pin until it is 1/8 inch thick. Use a cookie cutter to cut desired shapes. Place cut shortbread on a cookie sheet lined with parchment paper and preheat the oven to 350F. Place shortbread in the freezer while you make the soak.
In a 2-quart saucepan, add 2 cups of water, baking soda, and the barley malt syrup. Bring to a boil over medium heat, let boil for 2 minutes, then remove from heat. Transfer the soak to a shallow heatproof container. Dip each cookie, making sure to coat the entire cookie in the soak. Shake off the excess soak and place on the cookie sheet, leaving at least 2 inches between each cookie. Sprinkle with pretzel salt. Bake at 350°F for 10–11 minutes, until cookies are lightly browned. Remove from oven and let cool.
In a glass measuring cup or heatproof bowl, combine the cream, butter, and salt. Warm in the microwave or in a saucepan just until the butter is melted.
In a 2-qt saucepan, combine the granulated sugar and syrup. Cook over medium heat until the mixture turns a deep amber color (about 305–310° on a thermometer, or judge by color). Slowly add the warm cream mixture a little at a time, being very careful, as the caramel will bubble and steam vigorously. Whisk or stir until smooth. Continue cooking until the caramel reaches a copper color (245–250°). Remove from the heat immediately and transfer to a heatproof spouted cup for easy pouring. Let cool until slightly thickened but still pourable, about 15–20 minutes. If you wait too long it will be a bit hard to spread and will need to be gently reheated.
To assemble, spoon or spread 1 teaspoon onto each base cookies and top with another cookie. Repeat.
Cookies will stay fresh in an airtight container in the refrigerator for up to 10 days! Unbaked dough can be wrapped in plastic wrap and stored in the freezer for up to 3 months.
