Can I get a chocolate drink stain...

If the t-shirt is washable, you can try soaking the area with a few drops of liquid hand dishwashing detergent and a few drops of ammonia, and then blotting the stain gently with a moistened cloth. Rinse and repeat as necessary.

If still no results, try wetting the stain with a 3-percent solution of hydrogen peroxide to which 1 to 2 drops of ammonia have been added. Let sit for a minute or two. Rinse with water and blot dry. (Test this solution on an inside seam first before working on the stain itself.)