Break the white chocolate into small pieces or squares.
Add butter, white chocolate and mini marshmallows to a large pan. On a low to medium heat stir the mixture until smooth and melted. Once melted, remove from the heat. (To melt in the microwave see notes).
Pour cornflakes into the pan (or large mixing bowl if you warmed the mixture in the microwave). Mix thoroughly until evenly combined.
Spoon the cornflake mixture evenly between 24 cases and chill in the fridge for 1 hour or until set.
Video
Notes
You can also melt the mixture in the microwave on a low to medium heat. Stop the microwave every 10 to 15 seconds and give the mixture a thorough stir.