Prepare the Brownie Layer:
Preheat the oven to 180°C (350°F) and line a square baking tin (around 9×9 inches) with parchment paper.

Make the Brownie Batter:
Melt the butter and chocolate together in a heatproof bowl over simmering water (double boiler method) or in the microwave. Stir until smooth.
Whisk the eggs and caster sugar in a separate bowl until thick and pale. Gradually fold in the melted chocolate mixture.
Sift the gluten-free flour and cocoa powder together, then fold it into the egg mixture until just combined.
Add the chocolate chips and mix gently.

Bake the Brownie Base:
Pour the brownie batter into the prepared tin and spread evenly. Bake in the preheated oven for 20-25 minutes, or until a toothpick comes out with a few moist crumbs (not wet batter). Let the brownies cool completely in the tin.

Prepare the Caramel Layer:
In a saucepan over medium heat, melt the butter. Add the brown sugar, golden syrup, condensed milk, and salt. Stir constantly until the mixture begins to bubble. Let it simmer for 5-7 minutes, stirring occasionally, until it thickens and becomes a caramel consistency.

Assemble the Brownies:
Pour the caramel layer over the cooled brownie base, spreading it evenly. Place the tin in the fridge for 1-2 hours to allow the caramel to set.

Prepare the Top Chocolate Layer:
Melt the milk chocolate and white chocolate together in separate bowls over a double boiler or in the microwave. Once melted, pour the milk chocolate over the caramel layer and spread evenly. Drizzle the white chocolate over the top and swirl it gently with a knife to create a marbled effect.