Laura Cunliffe put four-month-old Mowgli in the microwave for five minutes after it attacked her pet goldfish (stock image used above).
The kitten was still alive when she removed it, but it died shortly afterwards.
She took the dying creature to a relative's house but Mowgli could not catch her breath and died an hour and a half later.
Brian Orsborn from the RSPCA said that Laura disclosed what she had done three days later when she visited Barnsley hospital.
The RSPCA tracked down the man asked to bury the cat, and they were then able to determine what had happened to the animal.
Lynsey Harris, RSPCA deputy chief inspector said: 'In the 13 years I have been in the job I have never dealt with a case like this before.'
'It is particularly horrendous because of the period of suffering for the kitten which would have been awful.'
'In the 13 years I have been in the job I have never dealt with a case like this before.'
'The death of the cat would have been prolonged and it is unimaginable what it would have gone through.'
Cunliffe, from Hoyland near Barnsley, admitted causing unnecessary suffering to an animal and was granted unconditional bail. She will be tried in March.