1. Line a 20cm (8”) square tin with a strip of parchment or foil, with the excess hanging over the sides of the tin to make removing the bars easier later on.
2. To make the bars place the flour, oats, salt and raisins into a large bowl and mix together until everything is evenly mixed. Place the butter, golden syrup and brown sugar into a saucepan and cook over medium heat, stirring every now and then until everything is melted and smooth. Add the milk powder and whisk until combined. Take the pan off the heat and add in the baking soda and 1 tbsp water, stirring together for a minute until the mixture is a little foamy. Pour this mixture over the oat mixture and mix together until well combined. Whilst the oat mixture is still warm tip it into our prepared pan and press into a flat and even layer.
3. Bake in the preheated oven for 20-25 minutes or until the flapjack is golden brown and just a little darker around the edges. Allow to cool in the pan whilst you make the caramelised topping.
4. Place the chocolate onto a rimmed baking tray (don't use a black or non-stick tray) and place into an oven preheated to 120C, 250F, Gas 1/2. Bake for 40-50 minutes. As it bakes remove every 15 minutes and stir thoroughly, making sure the chocolate is smooth and silky. Remove from the oven when the colour reminds you of dulce de leche. Pour the melted chocolate into a small bowl and pour in the cream, stirring to combine. Pour the ganache over the cooled flapjack bars and spread into an even layer. Pop the bars into the fridge to allow the ganache to set. Before its fully set sprinkle the ganache with the sea salt.
5. Cut into small bars and serve.