Hand it to Copper Vanurp in Glenden Wood for 20% of the xp toward your next point of Cooking skill (10 million max).
You give Copper Vanurp Ebon Gromnie Tooth Brush. Copper Vanurp tells you, "Wow! It's obvious you have spent a lot of time perfecting the noble art of Toothbrush crafting." You've earned X experience in your Cooking skill.