Preheat your oven to 180°C, or 160°C if you're using a fan oven, which is Gas Mark 4. Grease your baking tin and line it with parchment paper so the brownies lift out cleanly later.
Combine the butter and dark chocolate in a saucepan over low heat, or melt them in the microwave in short 20-second bursts. Go slowly here, because chocolate can burn quickly if you rush it. Stir until the mixture is completely smooth.
In a small bowl, mix half of the caramel with the flaky sea salt until it loosens slightly.
In a large bowl, combine the remaining caramel with the sugar and eggs. Beat everything together with an electric mixer or a whisk until it looks pale and slightly fluffy. This step builds the fudgy texture brownies are known for.
Pour the melted chocolate and butter into the egg mixture and stir until fully combined. In a separate bowl, sift together the flour and cocoa powder, then fold this into the wet ingredients until the batter turns smooth and glossy.
Spread half of the brownie batter into your prepared tin, smoothing it out to the edges. Drizzle half of the salted caramel over the top in long stripes using a teaspoon.
Spoon the rest of the batter over the caramel layer and smooth it gently, being careful not to drag the caramel underneath too much. Top with the remaining caramel in stripes across the surface.
Drag a skewer through the batter in swirling motions to marble the caramel through the brownies. Bake for 30-35 minutes, until the top looks slightly crusty and the center still has a gentle wobble when you shake the tin.
Let the brownies cool completely in the tin before cutting them into squares. I know it's tempting to dig in right away, but cooling helps the caramel set so your slices hold together properly.