Preheat oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper.
In a bowl, combine flour, butter, and sugar. Mix until a crumbly dough forms. Press firmly into the prepared pan.
Bake for 20-25 minutes, or until lightly golden.
While the shortbread is baking, prepare the caramel. In a saucepan, combine sweetened condensed milk, butter, brown sugar, and golden syrup. Cook over medium heat, stirring constantly, until the mixture thickens and turns a golden brown color (about 10-15 minutes).
Pour the caramel over the baked shortbread. Bake for another 15-20 minutes, or until the caramel is bubbly.
Let cool completely.
Melt the chocolate and vegetable oil in a double boiler or microwave. Spread evenly over the caramel layer.
Let the chocolate set completely before cutting into squares.
Notes
For a salted caramel flavor, sprinkle a pinch of sea salt over the caramel layer before baking.