Step 1: Preheat the Oven

Preheat your oven to 180°C (350°F) and grease and line a 23cm square baking tin with parchment paper.

Step 2: Make the Salted Caramel

In a saucepan over medium heat, melt the caster sugar, stirring continuously until it becomes a golden-brown caramel. Once melted, carefully add the butter and double cream, stirring constantly until smooth. Remove from heat and stir in the sea salt. Set the caramel aside to cool slightly.

Step 3: Melt the Chocolate and Butter

In a heatproof bowl set over a pot of simmering water (double boiler), melt the dark chocolate and butter together, stirring occasionally until smooth and glossy. Remove from the heat and allow it to cool slightly.

Step 4: Prepare the Brownie Batter

In a large mixing bowl, whisk the eggs, caster sugar, and light brown sugar until pale and fluffy. Add the vanilla extract. Gradually pour in the melted chocolate mixture, whisking until well combined.

Step 5: Sift in the Dry Ingredients

Sift the plain flour, cocoa powder, and baking powder into the bowl and gently fold them into the chocolate mixture until just combined. Be careful not to overmix, as you want the brownies to stay fudgy.

Step 6: Layer the Brownie and Salted Caramel

Pour half of the brownie batter into the prepared baking tin and spread it evenly. Drizzle half of the salted caramel over the brownie layer. Pour the remaining brownie batter on top and spread it evenly, then drizzle the rest of the caramel over the top. Use a knife or skewer to gently swirl the caramel into the brownie batter for a marbled effect.

Step 7: Bake the Brownies

Place the baking tin in the preheated oven and bake for 25-30 minutes, or until the brownies are set but still slightly gooey in the center. A skewer inserted should come out with a few moist crumbs, but not wet batter.

Step 8: Cool and Serve

Once baked, remove the brownies from the oven and allow them to cool in the tin for at least 30 minutes before cutting into squares. This will help them set and make cutting easier. Serve with an extra sprinkle of sea salt flakes on top, if desired.