We have the answer!

Clean copper pans with vinegar and salt.

Remove stuck toffee from pan by adding water and boiling the sugar away.

Salt, or sodium chloride, combines with acetic acid from the vinegar to produce sodium acetate and hydrogen chloride. Hydrogen chloride is a strong acid and the combination of it and sodium acetate rapidly cleans the surface.

The cleaning process leaves a very pure metal surface which quickly corrodes when exposed to the water, oxygen, and carbon dioxide from the air.

Good luck!