Translation:The boy has a chocolate cookie.
"Un biscotto di cioccolato" would be interpreted as a biscuit made of chocolate, but that would be a "cioccolatino"; "un biscotto al cioccolato" means a biscuit flavored with chocolate,and as such makes more sense. Prepositions can't always be directly translated between languages, unfortunately.
The preposition "AL" when talking about food doesn't mean "to the", it expresses the cookie was made "with a chocolate style" thus, it is a chocolate cookie (not the exact translation but it is hard to explain). Also in Spanish and other romance languages it is the used preposition. Other examples could be "pollo A LA mexicana (spanish), penne AL burro (italian), pato a l'orange (french) "
Dear Duo... with little time left in my day I just want to get all exercises correct... when I try to hurry I make silly mistakes... and get frustrated because I should know better.... but then when I get a wrong for a correct answer, I am even more frustrated. a chocolate biscuit and a chocolate cookie are the very same thing
My Italian-English dictionary says that chocolate is a feminine word spelt: la cioccolata. Linguee.com says it can be either (https://www.linguee.com/english-italian/translation/chocolate.html). Does anyone know why to use feminine or masculine?