Optional Step – Toasting the Cornflakes:
Preheat oven to 325°F and line 2 baking sheets with parchment paper.
Divide the cornflakes between the 2 pans.
Toast in the oven, turning occasionally for even browning, about 10 minutes.
Cool for 5 minutes, then transfer to a large bowl.
Main Preparation:
Line a 9×9 baking dish with baking spray and parchment paper. Set aside.
In a large pot, melt butter on low heat until it turns light brown and has a nutty aroma (4 to 6 minutes).
Add salt and whisk to combine.
Stir in marshmallows with a rubber spatula until fully melted.
Mix in the (toasted) cornflakes until well combined.
Transfer mixture to the prepared dish. Use a greased spatula to press and smooth the top.
Let cool for 30 minutes before removing from the pan. Slice and serve.