Preheat oven to 160 degrees celsius (fan-forced). Line a 20cm square baking tin with baking paper and set aside.
Using hand-held beaters or a stand mixer, cream the softened butter and sugar until pale and fluffy.
Add the vanilla extract and beat through.
Sift over the plain flour and baking powder and beat until the mixture has come together (it will be crumbly at first).
Press two-thirds of the mixture firmly into the base of the prepared tin and smooth with a spoon. Prick all over the base with a fork.
Bake for 10 minutes or until lightly golden.
Meanwhile, make the filling by placing the golden syrup, sweetened condensed milk, butter and a pinch of salt into a small saucepan over low heat. Stir continuously for 10-15 minutes or until the caramel has thickened.
Pour the caramel over the cooked base and sprinkle over the remaining shortbread mixture.
Return the slice to the oven for a further 15-20 minutes or until golden and firm to the touch. Remove from the oven and set aside until cool. Place in the fridge once cooled and before cutting.
