You do not have to declare chocolate in Japan.
Chocolate is considered a processed food and is generally allowed into the country of Japan without needing to declare it.
Although you don't need to declare the chocolate in Japan, you should avoid bringing in any prohibited items to Japan, like meat products as well as vegetables and fresh fruits as they are subject to strict regulations in Japan.
Processed foods like chocolate, chocolate cookies, canned goods, packaged foods and other packaged foods are often allowed into Japan without you needing to declare them.
But it's also essential that you declare any agricultural or meat products, like raw meat or fresh produce, because these foods are restricted and also require a proper inspection to be brought into Japan.
If you don't declare the agricultural or meat products like the raw meat or fresh produce, you could face penalties, if you don't declare them and they could be confiscated.
Declaration is also a formality in Japan, and when bringing the food items like chocolate or snacks into Japan, you don't have to be concerned about being fined or penalized for bringing them.
Because declaration of food items into Japan is a formality, you're more likely to be scrutinized for other prohibited items.
If you do need to declare something in Japan, be honest on the declaration form and declare any items that you're unsure about.
It's always best to declare an item in Japan if you're unsure, instead of risking being fined for that item for not declaring it.