Grease and line a 20cm square tin (I used a small lamington tray). Preheat oven to 170C.
Add flour and sugar to a bowl and rub in 175g of butter. Clump the dough together to make a ball. Press this mixture into the tin and smooth down. Then prick with a fork and cook for 5 mins.
Lower the oven to 150C and cook for a further 30-40 mins until pale golden and no longer doughy. Allow to cool in the tin.
Melt the remaining 200g of butter in a large microwaveable jug.
Add the condensed milk and golden syrup. Whisk the mixture until the butter is fully incorporated.
Heat in the microwave for 6-7 minutes, stirring every minute until the mixture is boiling. It is ready when it has thickened and turned a light golden brown. Pour the mixture over the shortbread base. Leave to set.
Melt the chocolate and pour over the set caramel mixture. Let cool and refrigerate.
Nutritional information is an estimate and provided as a courtesy. The values may vary according to the ingredients and tools that are used. Please use your preferred nutritional calculator for more detailed information.