Preheat your oven to 350F/180C bake. Grease and line a small baking tray with baking paper.
Add the cornflakes to a large heat-proof bowl and set aside. Make sure they are level cups, not heaped.
Add the butter, sugar and 1 tsp flaky sea salt to a medium pot and place over medium heat. Cook for 1-2 minutes, stirring, until the butter is melted.
Bring to a simmer and simmer for a further 1-2 minutes on medium-low heat, until glossy and the sugar has dissolved.
Remove from heat and pour the mixture immediately over the cornflakes. Toss together until well coated.
Tip the mixture out onto the prepared baking tray and spread into an even layer (about 1.5cm thick).
Bake on the upper-middle oven rack for 10-15 minutes, until golden brown and caramelized. Set aside to cool completely, before breaking up into pieces.
Store in an airtight container.
Dip the pieces of brittle in some melted, good-quality milk chocolate.
Transfer to a lined tray or board, sprinkle over some flaky salt and set aside in the fridge to set.
