Preheat the oven to 190°C (170°C Fan) and line a 9x9 inch square tin with parchment paper.

In a medium bowl, whisk together plain flour, bicarbonate of soda, salt, and cornflour.

Melt unsalted butter in a saucepan and then whisk in granulated and light brown sugar, followed by egg and vanilla extract.

Mix the dry ingredients into the wet until a thick dough forms, then fold in the chocolate chips.

Press the dough into the lined baking tin and bake for 18-22 minutes until edges are golden.

In another saucepan, combine condensed milk, unsalted butter, caster sugar, and golden syrup for the caramel.

Boil the caramel mixture for 5-7 minutes until thickened and golden.

Pour the caramel over the cooled cookie base and let it set for 1-2 hours.

Melt the chocolate types, drizzle them over the caramel, and create a marbled effect.

Once set, lift the bars from the tin, slice them, and serve.