Line an 20 x 20 cm (8 x 8 inch) baking tin with parchment paper with enough parchment to hang over the edges of the tin. Set the tin aside. Add the cornflakes to a large bowl and using your hands, crush the cornflakes into smaller pieces. Also set aside.
Add some water to a medium saucepan and place on a high heat. Allow the water to begin boiling then reduce to a low heat. Place a heat proof bowl over the saucepan ensuring the bottom of the bowl isn’t touching the water. If the bottom of the bowl is touching the water then carefully remove some of the water. To the bowl, add the butter, milk chocolate and dark chocolate then gently stir until melted. You can also melt in the microwave.
Once melted, remove the bowl from the saucepan and pour the cornflakes into the bowl and stir until all the cornflakes are coated in chocolate. As you’re stirring, crush the cornflakes into the chocolate to make the cornflake pieces even smaller.
Once all the cornflakes are coated, tip the mixture into the tin and use a spatula to press and smooth it out. Press the mixture tightly into the tin and corners. Place the chocolate cornflakes in the fridge for 2 hours then remove from the tin using the parchment paper overhang. Cut into 16 squares by cutting the whole thing into 4 pieces then cutting each piece into 4 again. Optionally, sprinkle with sea salt flakes.