Preheat oven to 180°C. Line a baking dish with baking paper (about 20x20cm).
Melt butter in a large bowl in the microwave with first measure of golden syrup. Once melted, stir through biscuit mix. Press 3/4 of the mix into the base of the baking dish, compacting with a spoon or bottom of a glass.
Place condensed milk and second measures of butter and golden syrup into a small pot on low heat. Cook for about 5 minutes, stirring constantly, until deeper in colour and thickening. Don't worry if you get some lumps.
Top base with caramel and remaining biscuit crumb. Bake for 30-40 minutes, until golden. Allow to cool completely before slicing.