Preheat oven to 300°F. Line a 9×13 baking sheet with parchment paper.

Place 3 cups of cornflakes into a large heat-safe bowl.

In a medium saucepan over medium heat, add butter, brown sugar, corn syrup, and vanilla.

Stir for 1–2 minutes until butter melts, then bring to a gentle boil. Reduce heat to low and stir for another 1–2 minutes until the mixture is smooth and glossy.

Remove from heat and immediately pour over the cornflakes. Toss to coat evenly.

Spread the coated cornflakes into an even 1/2–3/4 inch layer on the prepared baking sheet.

Bake for 20–30 minutes or until golden brown and caramelized, watching carefully to prevent burning.

Let cool completely on the tray.

Melt chocolate chips and drizzle over cooled brittle. Sprinkle with flakey sea salt.

Once the chocolate is set, break into pieces and serve or store in an airtight container.