Method

1. Preheat the oven to 170°C. Line a 12 inch squared brownie tin with greaseproof paper. Put the flour and sugar into a bowl and rub in 200g of the butter. Its texture will be sandy but will be malleable enough to form a ball shape when pushed together. Press this sandy shortbread mixture into the tin and smooth it with your knuckles. Prick it with a fork and cook for 5 minutes, then lower the oven to 150°C and cook it for a further 30mins or until it is pale golden and no longer doughy. Let it cool in the tin.
2. Melt the remaining 115g of butter in a saucepan on a low heat for 2-3 minutes, then add the condensed milk, golden syrup and rosemary. Whisk the mixture well until the butter is thoroughly incorporated. Bring to a slow simmer then keep the temperature even. Cook for 10 mins stirring continuously. This mixture can burn very easily so never take your eyes off it! It's ready when it's thickened and turned a light golden brown. Very gently remove the rosemary from the pan, then evenly pour the sauce over the cooled shortbread, sprinkle with the salt and leave it to set.
3. Break the chocolate into pieces and place in a microwaveable bowl. Microwave for 1min 30 seconds stirring halfway through until fully melted. Leave to cool for 5 mins then pour and spread the melted chocolate evenly over the toffee mixture and leave it to cool in the fridge for 1 hour. Once set, cut the caramel shortbread into 12-16 squares. The squares can be stored in the fridge to keep but if you let them sit out for a while the caramel goes lovely and gooey.