Preheat the oven to 350°F (175°C) and prepare a 9x13 inch baking pan by lining it with parchment paper.
In a medium saucepan, combine the unsalted butter and dark chocolate chips over low heat, stirring until melted and smooth. Remove from heat and let cool.
In a large mixing bowl, whisk together the granulated sugar, eggs, and vanilla extract until well combined and slightly frothy.
Gradually pour the cooled chocolate mixture into the sugary egg mixture, whisking constantly to prevent cooking the eggs.
In a separate bowl, sift together the all-purpose flour, cocoa powder, baking powder, and salt.
Fold the dry ingredients into the wet mixture using a spatula until just combined.
Pour half of the brownie batter into the lined baking pan and spread evenly. Drizzle half of the salted caramel sauce over this layer, then spread the remaining brownie batter on top.
Using a knife or skewer, gently swirl the remaining salted caramel sauce into the brownie batter.
Bake in the preheated oven for 25-30 minutes, checking for doneness with a toothpick.
Cool the brownies in the pan on a wire rack, then lift out using the parchment paper and cut into bite-sized squares. Drizzle with additional salted caramel sauce and sprinkle with sea salt flakes before serving.
Notes
For best results, let the brownies cool completely before cutting.