Preheat oven to 350 degrees F. Line a 9x13 inch baking dish with aluminum foil or parchment paper. Spray paper with nonstick spray.
Place flour, sugar, salt, and butter in a food processor and pulse until mixture resembles wet sand. Press the shortbread mixture into prepared pan.
Bake until lightly browned, about 20 minutes.
Remove from oven.
CARAMEL
In a heavy-bottomed saucepan over low heat, combine sweetened condensed milk and butter. slowly bring mixture to a low boil, stirring constantly. Continue stirring until the mixture becomes thick.
This will take about 15 minutes. Stir constantly and do not walk away, caramel scorches easily.
Pour caramel over the cooked shortbread and spread evenly with an offset spatula.
Immediately top with crushed pretzels and gently press pretzels into caramel.
Melt chocolate in the microwave at 30-second intervals until completely melted and smooth. Stir between each 30-second interval.
Transfer chocolate to a zip-top bag.
Snip corner off bag and drizzle chocolate over bars.