Preheat the oven to 300 degrees Fahrenheit, line a baking sheet with parchment paper, and set aside.
Add the butter, sugar, and corn syrup to a medium saucepan and cook over medium heat until the sugar and butter are fully melted.
Stir until the mixture boils. Reduce the heat to low and cook for 2 minutes. After 2 minutes, remove the pan from the heat
Add the cornflakes to a large heat-safe bowl and pour the sugar mixture over them. Immediately stir the cornflakes until they are evenly coated.
Pour the cornflake mixture onto the prepared baking sheet and bake for 30 minutes. Remove from the oven and let it cool completely.
Once cooled, gently melt the chocolate in the microwave at 50% power in 30-second increments, stirring between each. Once fully melted, drizzle the chocolate over the cooled brittle. Let the chocolate set completely before cracking into pieces for serving.