Preheat the oven to 350°F (175°C). Line an 8-inch square metal baking pan with parchment paper, leaving some overhang on two sides for lifting the brownies out later.

Place the chopped semisweet chocolate and melted butter in a heatproof bowl. Microwave in 20-second intervals, stirring after each, until the chocolate is almost completely melted. Stir until smooth, then set aside to cool slightly. The mixture should be warm but not hot.

In a large bowl, whisk the granulated sugar, brown sugar, eggs, and vanilla for about 1 minute, until smooth and slightly thickened. Do not whip the mixture until fluffy; a dense batter helps produce fudgy brownies.

Slowly whisk the warm chocolate-butter mixture into the egg mixture until evenly combined. The batter should look glossy and smooth.

Whisk together the flour, cocoa powder, and salt in a separate bowl. Add them to the chocolate mixture and fold with a rubber spatula just until no dry streaks remain. The batter should be thick and glossy. Stop mixing as soon as the flour disappears.

In a small bowl, stir 1/2 cup (160 g) caramel sauce with 1 tablespoon (8 g) flour until smooth. The mixture should remain thick enough to sit on top of the brownie batter rather than immediately running into it.

Spread about two-thirds of the brownie batter evenly into the prepared pan. Spoon roughly half of the caramel mixture over the surface in small dollops. Add the remaining brownie batter and gently spread it into an even layer.

Add the remaining caramel in small dollops across the top. Drag a wooden skewer or butter knife through the caramel and brownie batter in a loose zigzag pattern. Do not over-swirl, or the caramel will blend into the batter instead of forming distinct ribbons.

Bake for 28 to 33 minutes, until the edges are set and slightly puffed while the center still looks moist. A toothpick inserted into the brownie portion should come out with several moist crumbs rather than wet batter. Avoid inserting it directly into a caramel pocket when checking doneness.

Place the pan on a wire rack and cool for at least 1 hour. For especially clean slices, refrigerate the cooled brownies for 20 to 30 minutes before cutting.

Use the parchment overhang to lift the brownies from the pan. Cut into 16 squares with a sharp knife, wiping the blade between cuts if needed. Serve at room temperature for the softest texture.