Preheat oven to 180°C (160°C fan-forced) and lightly grease an 21cm square tin and line with baking paper.
For shortbread, beat butter, peanut butter and sugar together, using electric beaters, until light and fluffy. Carefully fold in flours, stirring with a wooden spoon until mixture is well combined.
Spread mixture into the base of the prepared tin, pressing it down firmly to smooth the surface. Bake in preheated oven for 15 minutes or until lightly golden. Remove from oven and allow to cool.
For caramel filling, combine butter, condensed milk, golden syrup and salt together, stirring continuously until butter melts. Bring to a boil and cook for 5 minutes or until mixture thickens slightly, stirring occasionally. Pour caramel sauce over the cooled shortbread base and place in oven for 15 minutes or until caramel sets and becomes golden. Remove from oven and cool before chilling completely in the refrigerator for 3-4 hours.
For the ganache, combine chocolate and cream together in a small bowl over a saucepan of simmering water, stirring occasionally, until chocolate melts. Remove from heat and allow to cool. Pour over chilled caramel, smoothing top with a spatula. Allow chocolate to set before cutting into 24 pieces.