Line your pan with parchment paper, leaving some overhang for easy removal.
In a microwave-safe bowl, melt the dark chocolate in 30-second bursts, stirring each time until smooth and glossy. Set aside.
In a small saucepan, gently melt the butter and golden syrup over low heat, stirring until combined and fragrant. Remove from heat and stir in vanilla extract if using.
Pour the melted chocolate into the saucepan with the warm butter and syrup. Stir quickly until everything is well blended into a shiny, smooth mixture.
Add the cornflakes gradually into the chocolate mixture, folding gently with a spatula to coat each flake evenly. Be careful not to crush them.
Transfer the mixture into your prepared pan. Using a spatula or damp hands, press the mixture firmly and evenly into the pan, smoothing the top for a flat surface.
Sprinkle a pinch of sea salt flakes on top for that perfect sweet-salty contrast.
Place the pan in the fridge and chill for at least 2 hours, or until the bars are firm and set.
Once chilled, lift the block out of the pan using the parchment overhang. Slice into 12 even squares with a sharp, hot knife for clean edges.
Arrange the bars on a plate or store in an airtight container in the fridge. These are best enjoyed within a week for maximum freshness and crunch.
