Easy-Off oven cleaner. Spay on and let sit and bubble for half an hour. Do it outside as the fumes are toxic.

You will probably need to repeat it a few times to get it all off. But then once its off, you will have to polish the surface really well to remove any imperfections.

I did a stem once that was anodized black. Once I was finished with the oven cleaner, the stem had lots of imperfections in it that would not buff out. Anodizing covers lots of blemishes. I still use the stem today on my touring bike though.

Have fun.