Preheat your oven to 180°C (350°F) and line a 20x20 cm (8x8-inch) baking pan with parchment paper, making sure to leave some overhang for easy removal.

To prepare the pretzel bottom, crush the pretzels using a food processor or by placing them in a zip-top bag and rolling over them with a rolling pin until finely ground.

In a mixing bowl, combine the crushed pretzels with the granulated sugar and melted butter, stirring until the mixture is fully moistened and well blended. Transfer this mixture to the prepared baking pan and press it evenly across the bottom to form a firm crust. Place the pan in the refrigerator while you prepare the brownie batter.

For the brownies, melt the butter in a saucepan over low heat. Remove from heat, add 200 grams of chopped dark chocolate, and stir until the chocolate is completely melted and smooth. Set aside to cool slightly.

In a separate bowl, whisk together the flour, sifted cocoa powder, and salt until combined.

Using an electric mixer on high speed, beat the eggs, both granulated and brown sugars, and vanilla extract for about 5 minutes until the mixture becomes pale, thick, and fluffy. Pour in the cooled chocolate-butter mixture and beat for another minute until incorporated.

With a rubber spatula, gently fold in the dry ingredients until just combined. If using, fold in the chopped dark and milk chocolate pieces. Avoid overmixing; the batter will be very thick.

Remove the pan with the chilled pretzel crust from the fridge and spread the brownie batter evenly over the top. Bake in the preheated oven for about 25 minutes, checking for doneness around 20–30 minutes depending on your oven. The brownies are done when a toothpick inserted in the center comes out with a few moist crumbs but no wet batter. Let the brownies cool slightly while you prepare the caramel.

To make the caramel, heat the granulated sugar in a medium saucepan over medium heat, stirring constantly with a wooden spoon. The sugar will first clump and then gradually melt into an amber liquid. Once fully melted, immediately add the butter and whisk carefully—it will bubble vigorously. Continue stirring until the butter is completely incorporated. Slowly pour in the heavy cream while whisking, being cautious of the bubbling. Whisk until the mixture is smooth and allow it to boil gently for 1 to 2 minutes.

Let the caramel cool for about 5 minutes, then pour it evenly over the warm brownies. Spread into an even layer and allow the entire dessert to cool completely for at least 6 to 7 hours, preferably overnight, before cutting into squares and serving. For an extra touch, sprinkle with flaky sea salt just before serving.