Brown the butter and transfer to a microwave-safe bowl.
Add the 240g marshmallows on top of the browned butter and the Rice Krispies.
Microwave in 30–40 second bursts, stirring between each.
Heat until the marshmallows are fully melted and the mixture is evenly coated and glossy.
Add salt and fold in the chopped ChocoMagic cookies.
Line a small tin.
Press mixture in gently.
Cool for 20–30 minutes before slicing.