Step 1

Adjust an oven rack to the lower-middle position and heat the oven to 350 degrees F. Spray an 8-inch square baking pan with nonstick cooking spray. Line the pan with overlapping pieces of foil and spray the foil.

Step 2

In a 3-quart saucepan, heat the sugar over medium-high heat, whisking as the sugar begins to melt. Some of the sugar will harden into clumps, but that's okay - it will melt eventually - just keep whisking. Continue to cook the sugar until it reaches a dark amber color. At that point, whisk in the salt, and then add the butter all at once and whisk until it is completely incorporated into the sugar. Remove the pan from the heat and pour in the heavy cream (it will foam up when first added). Continue to whisk until it forms a smooth sauce. Allow to cool for 10-15 minutes before using in the brownies. The leftover sauce can be stored, covered, in the refrigerator for up to 2 weeks. (You'll probably need to warm it up a bit straight from the refrigerator.)

Step 3

In a medium heatproof bowl set over a pan of almost-simmering water, melt the chocolates and butter, stirring occasionally until smooth. (Or, melt in the microwave on 50% power for 30-second increments, stirring after each, until melted and smooth.) Whisk in the cocoa until smooth. Set aside to cool.

Step 4

Whisk together the eggs, sugar, vanilla, and salt in a medium bowl until combined, about 15 seconds. Whisk the warm chocolate mixture into the egg mixture; then stir in the flour until just combined. Pour about half of the brownie mixture into the prepared pan and spread into the corners. Spoon 9 dollops of Salted Caramel Sauce on top of the brownie batter. Top with the remaining brownie mixture, spread into the corners and level the surface with a spatula. Again, spoon 9 dollops of the Salted Caramel Sauce on top of the brownie batter. With a dull butter knife, gently run it through the batter to swirl the batter just a bit (don't do it too much or it will all mix together). Sprinkle a bit of sea salt on top.

Step 5

Bake until slightly puffed and a toothpick inserted in the center comes out with a small amount of sticky crumbs, 35 to 40 minutes. Cool on a wire rack to room temperature, about 2 hours. Remove the brownies from the pan using the foil handles and transfer to a cutting board. Cut into squares. Brownies can be stored in an airtight container at room temperature.