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.
Melt the chocolate by placing a heat proof bowl over a pot of boiling water. Ensure the bottom of the bowl isn’t touching the water. To the bowl, add the butter, milk chocolate and dark chocolate. Stir until melted. You can also melt in the microwave.
Pour the cornflakes into the melted chocolate and butter then 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 smooth it out and press tightly into the tin. Set in the fridge for 1 hour 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.