Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal.
In a medium saucepan, melt the butter and dark chocolate chips together over low heat, stirring until smooth. Remove from heat and let cool slightly.
In a large bowl, whisk together the granulated sugar, eggs, and vanilla extract until well combined and slightly frothy.
Gradually pour the melted chocolate mixture into the egg mixture while whisking continuously to prevent the eggs from curdling.
In another bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet ingredients until just combined. Be careful not to overmix.
Spread half of the brownie batter evenly in the prepared baking pan. Drizzle half of the salted caramel sauce over the batter, then carefully spread the remaining brownie batter on top.
Using a knife or a skewer, swirl the remaining caramel into the brownie batter for a marbled effect.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with moist crumbs.
Allow the brownies to cool completely in the pan on a wire rack. Once cooled, lift them out using the parchment paper and cut into bite-sized squares.
Drizzle additional salted caramel sauce over the top and sprinkle with sea salt flakes just before serving.
Notes
Allow brownies to cool completely for best texture.