Heat butter and marshmallows together in a pan stirring continuously until marshmallow is completely melted.
Pour over the rice krispies and stir well.
Push into chosen mould, (if you require the RKT to fit into intricate moulding you can blitz the dry rice krispies in a food processor on pulse to make it a bit finer before mixing it with the marshmallow)
or mould by hand and leave to set in fridge.
(when moulding by hand, wet hands and shake off excess water but do not dry)
Once set it can be carved into shape.