In a medium saucepan, heat the sugar on medium-high for about 5-6 minutes. Use a heat-proof rubber spatula to stir the sugar to make sure it doens't burn on the bottom. The sugar will get lumpy at first, then turn a golden brown color.
After the sugar has completely melted, there are no more clumps, and it's a nice dark amber color, remove from the heat and add in the butter. Be careful because it should rapidly bubble at first. Use your spatula to stir the mixture for 1-2 minutes, until the sugar and butter come together. If they seem like they are separating just continue to mix, it will eventually come together!
Put the mixture back on the heat and slowly add in the heavy cream while stirring, this should bubble as well. Let it boil/bubble for about 1 minute and then remove from heat, adding in the salt and mixing to combine everything together. Let it cool.
Make the brownies
Preheat oven to 350°. Line a 9×9 baking pan with parchment paper and set aside.
In a microwave-safe bowl, heat butter and 4oz of chocolate on high for 30 seconds. Mix and then heat in 10 to 15-second intervals (mixing in between) until the mixture is smooth and melted together. Once melted, add to a large bowl with the sugar and mix until combined. Add in the eggs + yolk, as well as the vanilla.
Add the dry ingredients- flour, cocoa powder, and salt. Mix until everything is combined and there are no streaks of flour left.
Pour a little over half the brownie batter into the pan, and then take ¾ cup of the caramel sauce and spread over top. You can use a knife to swirl it into the brownies, just be careful not to go all the way to the edge. Top with the rest of the brownie batter.
Bake for 30-35 minutes or until a knife inserted in the center of the brownies comes out clean. Allow the brownies to cool completely (about 1 hour), drizzle the remaining caramel sauce on top, and cut into 12 squares! Finish with some flaky sea salt and enjoy!