Step 1

Preheat oven to 180C/160C fan forced. Grease a square 20cm cake pan and line the base and sides with baking paper. Place the condensed milk, white chocolate and vanilla in a saucepan over medium-low heat. Cook, stirring, until melted and combined. Simmer, stirring constantly, for 5 minutes or until light golden in colour. Set aside, stirring occasionally, for 10 minutes to cool slightly.

Step 2

Meanwhile, place the butter and dark chocolate in a heatproof bowl over a saucepan of simmering water (don't let the bowl touch the water). Stir with a metal spoon until melted and combined. Remove from heat. Quickly stir in the egg, sugars, flour and salt until just combined.

Step 3

Pour the dark chocolate mixture into the prepared pan. Dollop over the fudge mixture. Use a knife to create a swirled effect. Bake for 30 minutes or until a skewer inserted into the centre comes out with moist crumbs clinging. Set aside in the pan to cool completely. Drizzle with caramel topping and cut into pieces.