Preheat oven to 180 °C (350 °F). Line 20x20 cm (8x8-inch) baking pan with parchment paper.
For pretzel bottom, grind the pretzels in a food processor or put them in zip-top bag and run rolling pin over the pretzels to crush them.
In a bowl, mix together the pretzel crumbs, sugar, and melted butter until everything is moistened and evenly mixed. Transfer the mixture to the prepared pan and press the crust mixture across the bottom of the pan. Put the pan in the fridge.
For brownies, melt the butter in a saucepan, remove from heat and add in 200 grams chopped chocolate. Stir until melted and completely smooth. Set aside to cool.
In a bowl whisk flour, sifted cocoa powder and salt.
With an electric mixer on maximum speed, whisk the eggs, both sugars and vanilla for 5 minutes until pale and fluffy. Pour in the cooled chocolate mixture and beat for 1 minute.
Using rubber spatula, fold in dry ingredients until incorporated. Fold in chopped chocolate if using. Don't over mix. Batter will be very thick. Remove the pan from the fridge and pour the batter over pretzel crust. Bake around 25 minutes (it can take 20-30 minutes, depending on your oven), or until toothpick inserted in the center comes out with few moist crumbs (but not wet batter). Leave to cool slightly until you make caramel.
For caramel, heat granulated sugar in a medium saucepan over medium heat, stirring constantly with a wooden spoon. The sugar will clump up first, but will eventually completely melt. Once sugar is completely melted, immediately add the butter and whisk until combined. Be careful because the mixture will bubble up. Stir the butter into the caramel until it is completely melted. Slowly pour the heavy cream into the caramel and whisk until incorporated. Allow the mixture to boil for 1-2 minutes.
Cool for about 5 minutes, then pour over brownies into even layer. Cool completely, for 6-7 hours, preferebly overnight before cutting and serving. If desired, sprinkle with flaky sea salt.