Preheat your oven to 350°F (175°C). Line your 9x9 inch baking pan with parchment paper, allowing some overhang for easy removal later.
In a double boiler or microwave-safe bowl, combine the unsalted butter and semisweet chocolate chips. Melt them together until smooth, stirring frequently to prevent burning.
In a separate bowl, whisk together the natural cocoa powder, all-purpose flour, baking powder, and salt. Set aside.
In a large mixing bowl, combine the light brown sugar and granulated sugar. Pour the melted chocolate mixture over the sugars, and mix until well combined.
Add the large eggs, egg yolk, and vanilla extract to the chocolate mixture. Whisk until everything is well incorporated.
Gradually add the dry ingredients to the wet mixture, folding gently until just combined. Be careful not to overmix; a few lumps are okay!
In a saucepan, combine the unwrapped caramels and heavy cream. Heat over low heat, stirring until the mixture is smooth and creamy. Remove from heat.
Pour half of the brownie batter into the prepared baking pan, spreading it evenly. Drizzle the caramel mixture over the batter, then top with the remaining brownie batter. Use a knife to swirl the caramel into the brownie batter for a marbled effect.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for about 10 minutes. Carefully lift them out using the parchment paper and transfer to a wire rack to cool completely. Once cooled, cut them into bite-sized pieces and sprinkle with fine sea salt before serving.
