Preheat your oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper, allowing some overhang for easy removal.
In a medium saucepan over low heat, melt the butter and chopped chocolate together, stirring frequently until smooth. Remove from heat and let cool slightly.
In a large mixing bowl, combine granulated sugar, brown sugar, eggs, and vanilla extract. Whisk until well blended.
Gradually add the melted chocolate mixture to the sugar-egg mixture, whisking continuously until fully incorporated.
In a separate bowl, sift together the flour, cocoa powder, and salt. Gently fold the dry ingredients into the wet ingredients using a spatula, being careful not to overmix.
Stir in the crushed pretzels, reserving a handful for topping.
Pour half of the brownie batter into the prepared pan and spread it evenly. Drizzle half of the caramel sauce over the batter.
Pour the remaining brownie batter over the caramel layer, then spread it out gently. Finally, drizzle the rest of the caramel sauce on top and swirl it slightly with a knife for a marbled effect.
Sprinkle the reserved crushed pretzels and a pinch of flaky sea salt over the top.
Bake in the preheated oven for 30-35 minutes, or until a toothpick comes out mostly clean with a few moist crumbs (not wet batter).
Allow the brownies to cool in the pan for about 10 minutes before lifting them out using the parchment paper. Let them cool completely on a wire rack before slicing into squares.
Notes
Serve the brownies on a white plate, drizzled with extra caramel sauce and sprinkled with flaky sea salt for an elegant touch.