Line an 8" or 20cm square baking pan with non stick paper and pour in the shortbread base. Using a metal spoon of flat cup press the base down into a flat even layer.
Bake the shortbread base in a 160C/320F oven for about 35 minutes or until the edges are just starting to brown. It will be soft but will continue to harden as it cools.
In a medium sized saucepan add the butter, sweetened condensed milk, sugar and golden syrup. Heat over medium to low heat for about 5 minutes, stirring continuously.