Preheat oven to 350°F and grease a 9 x 9-inch baking pan.
Combine crushed gluten free pretzels, sugar and melted butter. Press firmly into the prepared baking pan to create an even layer.
Bake for 8 minutes. Remove from the oven and allow to cool while preparing brownie batter.
Brownie
Place Gluten Free Brownie Mix in a large bowl. Add butter, egg and water, and mix with an electric mixer on medium speed until just combined.
Spread brownie batter evenly over the pretzel crust and smooth top.
Bake until a toothpick inserted in the center comes out mostly clean, about 25 minutes. Let cool completely before frosting.
Frosting
Combine sweetened condensed milk and chocolate chips in a heavy saucepan over low heat. Cook, stirring frequently, until chocolate chips are melted. Remove from heat.
Stir in vanilla extract and then spread mixture over warm brownies. Sprinkle with crushed gluten free pretzels and sea salt.