Chocolate stains are common, especially when you got kids : they love chocolate, whether milk, white or dark.
Often enough cold water will remove the chocolate stain.
But if the stain is embedded, scrape the dry chocolate and then dab with a cloth soaked in soapy water.
If the stain persists, wipe it with a cotton cloth soaked in benzene or diluted ammonia or white vinegar.
You can also use warmed glycerine to penetrate the stain. Remember to rinse the treated area afterwards with warm water.