It was claimed that the prototype knife would heat up to allow butter to be spread more easily.
The knife was also reported on by the UK Daily Mail, amongst other places.
The We Love Ad site talks about the success of this marketing campaign, describing this as
a one-off branded prototype butter knife with heated, battery-powered element operating at 48ºC, the optimum temperature to soften butter - and launched it via traditional and social media, with supporting infographic, YouTube video and ‘#toastieknife’ hashtag.
The advert brags about achieving an optimal butter spreading, at a temperature of 41.8°C in 30 seconds, using two AA batteries.
I'd be interested to know if a working prototype was ever produced.