Walnut Hollow Burner

I’m new to the whole wood burning thing, but decided to do cutting boards with my coworkers’ last names for Christmas. After using a letter, I can’t seem to figure out how to get the soot off of the stamp. If I don’t clean it off then the next time I go to use that letter, it’s burning the wood faster (and worse) because it has all of that leftover soot still on the stamp. What’s the best way to clean these after use?