Spray a 9x5 loaf pan with non-stick spray and line it with parchment paper so that there is overhang on either side. Set aside.
Add your shortbread cookies to a ziploc bag and go over them with a rolling pin so that they are crumbly but still have some small chunks remaining.
Add butter, honey, light brown sugar and cocoa powder to a saucepan and heat at medium heat, stirring gently until it has melted into a thick chocolate sauce.
Take it off the heat and add the crushed shortbread and raisins into the chocolate sauce, stirring so that everything is well coated in the chocolate.
Transfer the tiffin mixture to your prepared 9x5 loaf pan and smooth down.
Place into the fridge to set (about 2 hours).
When the tiffin has set, break up the chocolate and add it to a microwave safe bowl. Microwave in 30-second intervals, bringing it out to stir every 30 seconds until melted.
Pour the chocolate out over the tiffin and smooth it down.
Place into the fridge to set (about 20 minutes).
When the tiffin has set, remove it from the pan using the parchment paper overhang.
Cut into squares and serve.