Line your square pan with parchment paper, leaving overhang on the sides for easy removal. Set aside.
Break the digestive biscuits into small pieces, aiming for a mix of fine crumbs and some larger bits for texture.
In a heatproof bowl, melt the dark chocolate and butter together over a double boiler or in short bursts in the microwave, stirring well after each. You're looking for a glossy, smooth mixture that smells rich and cocoasty.
Pour the melted chocolate mixture into the crushed biscuits. Add the golden syrup and vanilla extract, then stir thoroughly until all the crumbs are coated and the mixture feels thick yet spreadable.
Transfer the mixture into your prepared pan. Use a spatula to press it down firmly and evenly, making sure the surface is smooth and compacted.
If desired, press extra biscuits or nuts onto the top for added crunch and presentation.
Place the pan in the fridge and chill for at least 2 hours, or overnight if possible, until the bars are firm and shiny on top.
Once set, lift the entire block out using the parchment overhang. Use a sharp knife to cut into squares or rectangles, wiping the knife clean between cuts for neat edges.
Serve immediately for a fudgy treat, or store in an airtight container in the fridge for up to 4 days. For longer storage, wrap tightly and freeze, then thaw in the fridge when ready to enjoy.
