We burnt a teabiscuit , buttered and can't get rid of the smell. I have tried both vinigar and baking soda

Have you tried a slice of lemon in a bowl? Put the lemon in a bowl with a bit of water and put inside the microwave and run it for around 5 minutes, more if you need to, it depends on the power of your microwave. This worked when my son accidentally put a bun in the microwave for 10 minutes instead of 10 seconds.

Good luck.

I was actually going to suggest half a lemen good call Alty xx