Preheat the oven to 180°C (160°C fan-forced, 350°F). Line a medium baking tray with baking paper.
In a medium pot, add the butter and brown sugar. Bring to the boil over medium-high heat and cook for 2 minutes until the sugar has dissolved and the mixture has a lava-like consistency.
Remove from the heat and add the vanilla extract. Stir to combine.
Add the cornflake cereal and toss well to coat every flake completely.
Decant the coated cornflakes onto the prepared tray, spreading them out in an even layer.
Bake for 10 to 12 minutes (11 minutes is ideal) until golden brown. Watch carefully in the last 2 minutes to prevent burning.
Remove from the oven and allow to cool completely in the tray.
Melt the chocolate in a double boiler. Alternatively, sprinkle chopped chocolate or chocolate chips directly over the hot cornflake crack, wait 2 minutes for it to melt, then spread.
Spread the melted chocolate evenly over the cooled cornflake base using an offset spatula.
Sprinkle flaky sea salt over the chocolate whilst still wet.
Allow the chocolate to set and harden completely (approximately 30 minutes at room temperature, or 15 minutes in the fridge).
Break into rustic shards of varying sizes.
Store in a sealed container at room temperature for up to 2 weeks.