Make the brownie layer:
Preheat oven to 180°C (350°F). Line a 9×9-inch (23cm) square baking tin with parchment paper.
Melt the butter and dark chocolate together in a heatproof bowl over a pan of simmering water or in the microwave in short bursts. Let cool slightly.
In a separate bowl, whisk the eggs and caster sugar until thick and pale. Gently fold in the cooled chocolate mixture.
Sift in the flour and cocoa powder, and fold gently. Stir in the chocolate chips.
Pour the batter into the prepared tin and bake for 25–30 minutes or until just set with a slight wobble in the center. Cool completely.

Make the caramel:
In a saucepan, add butter, sugar, golden syrup, and condensed milk. Stir continuously over medium heat until it thickens and turns a deep golden color—about 10–12 minutes.
Pour the caramel over the cooled brownie layer and smooth evenly. Let set in the fridge for at least 1 hour.

Add the chocolate topping:
Melt the milk chocolate and pour over the set caramel layer. Melt the white chocolate separately and drizzle or swirl into the milk chocolate for a marbled effect.
Chill until fully set—about 1 hour.

Slice and serve:
Use a hot knife to cut into squares or bars. Serve and enjoy!