Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan and line it with parchment paper, leaving some overhang for easy removal.
In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and well combined.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually fold this dry mixture into the wet ingredients until just combined – be careful not to over-mix.
Gently fold in the pretzel pieces, reserving a handful for topping if desired.
Pour half of the brownie batter into the prepared pan, spreading it evenly. Drizzle half of the salted caramel sauce over the batter.
Carefully pour the remaining brownie batter on top, spreading it evenly. Finally, drizzle the rest of the caramel sauce over the top and use a toothpick or knife to create swirls.
Sprinkle the reserved pretzel pieces and a pinch of sea salt flakes over the top.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan on a wire rack before lifting them out using the parchment paper. Cut into squares once completely cool.
Notes
Allow brownies to cool completely for best texture before cutting.