Line your baking pan with parchment paper, leaving overhang for easy removal. Set aside.
Place the chopped chocolate and butter in a microwave-safe bowl. Microwave in 30-second bursts, stirring well each time, until the mixture is smooth and glossy.
While the chocolate melts, measure out the cornflakes into a large bowl. If you like, lightly toast them in a dry skillet over medium heat until golden and fragrant, then let them cool.
Pour the melted chocolate mixture over the cornflakes. Use a spatula to gently fold and coat every flake evenly, being careful not to crush them. The mixture should become shiny and well-coated.
Transfer the coated mixture into your prepared pan. Press down firmly with the back of a spatula or your fingers to create an even, compact layer. Dampening your hands slightly helps prevent sticking.
Place the pan in the fridge for at least 2 hours, or until the bars are set and firm. For quicker setting, pop it in the freezer for about an hour, checking that it’s firm and crackly on top.
Once set, lift the whole block out of the pan using the parchment overhang. Slice into squares or bars with a sharp knife, wiping the blade between cuts for clean slices.
Serve the bars immediately or store them in an airtight container at room temperature for up to 3 days, or in the fridge for up to a week. Let them sit at room temp for about 10 minutes before serving for the best crunch.
