Step 1

Arrange a rack in the middle of the oven and heat to 350°F. Place the chocolate in a large bowl.

Step 2

Melt the butter in an 8- or 9-inch cast iron skillet over medium heat. Pour the melted butter over the chocolate, leaving a film of butter on the pan. Set the pan aside. Let the mixture sit for a minute, then stir to evenly combine — the chocolate should be melted and the mixture should be smooth.

Step 3

Add the sugar and whisk to combine. Whisk in the eggs, one at a time, followed by the vanilla. Whisk vigorously for about 1 minute. Stir in the flour and salt.

Step 4

Transfer the batter to the skillet and spread into an even layer. Drop dollops of the caramel sauce all over the top. Using a butter knife, gently swirl the caramel sauce into the batter, running the knife lengthwise and crosswise. Sprinkle the top evenly with the flaky sea salt.

Step 5

Bake until a tester inserted in the center comes out with just a few crumbs, 25 to 30 minutes. The edges should look firm and well-baked, the center should be moist but not gooey, and the caramel sauce should be bubbling.

Step 6

Cool completely in the skillet on a wire rack. Cut into squares and serve.