Preheat oven to 180 °C (350 °F). Line a 20×20 cm (8×8-inch) baking pan with parchment paper.
For the pretzel bottom, grind the pretzels in a food processor or crush them in a zip-top bag with a rolling pin.
Mix the pretzel crumbs, sugar, and melted butter in a bowl until evenly combined. Press the mixture into the bottom of the prepared pan. Refrigerate while preparing the brownies.
For the brownies, melt the butter in a saucepan, remove from heat, and stir in the chopped dark chocolate until smooth. Let cool slightly.
In a bowl, whisk together flour, cocoa powder, and salt.
In a separate bowl, beat eggs, both sugars, and vanilla extract with an electric mixer on high for 5 minutes until pale and fluffy.
Pour the cooled chocolate mixture into the egg mixture and beat for 1 minute. Gently fold in the dry ingredients until just combined. Optionally, fold in the chopped chocolates.
Pour the thick batter over the pretzel crust. Bake for 20-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Let cool slightly.
For the caramel, melt the sugar in a saucepan over medium heat, stirring constantly. Once melted, add the butter and whisk until combined, then slowly add the heavy cream and whisk. Let boil for 1-2 minutes, then remove from heat and cool for 5 minutes.
Pour the caramel evenly over the brownies. Let cool completely for 6-7 hours or overnight. Optionally, sprinkle with flaky sea salt before serving.