Melt the butter, mix with the dry ingredients.
Spread the mixture in a deep baking tray (lay a piece of parchment paper before putting in the mixture). Bake at 180c for 15-20minutes.
Put butter, golden syrup into a pot, after the butter melted, add in condensed milk, boil it with medium to low heat till is thickened. (make sure you whisk the mixture constantly)
Spread the caramel evenly on the baked shortbread.
Melt the chocolate and oil in a bowl, mix properly. Pour it on after the caramel cool down.
When the chocolate solidify, cut the slab into square pieces with hot blade.