Preheat the oven to 150°C and line a 23cm square cake tin with parchment.
Pulse flour and butter in a food processor until the mixture resembles fine breadcrumbs.
Add caster sugar and pulse until combined.
Press the mixture firmly and evenly into the prepared tin using a spoon and knuckles.
Bake for 30 minutes until light golden brown and set aside to cool.
Melt butter, condensed milk, and syrup in a saucepan until smooth.
Boil the mixture while stirring until it thickens into a golden-brown caramel.
Pour the caramel over the shortbread and allow it to cool completely.
Melt the chocolate over a pan of simmering water.
Pour chocolate over the caramel and let it cool completely before cutting into squares.