The wonderful thing about mochi is that you can put anything inside. If you need some inspiration you can go with chocolate ganache, ice cream, fruits, peanut butter, crushed cookies, jam, sweet bean/seed pastes, and even savory things. The only caveat is that you probably want whatever you are making to look good, so I would stick to foods that you know will keep the shape you want.
Red Bean Paste and Black Sesame Paste – you can get right out of a can (or make it yourself)
Sesame Peanut Butter – I didn’t have sesame paste in my house so I crushed sesame by hand with a mortar pestle and mixed it with peanut butter. I used about equal parts of both because I didn’t want a very sweet mochi (with too much peanut butter), and when you use more sesame it keeps it’s shape better.
Chocolate Ganache AKA Chocolate Truffles- It’s pretty straightforward to make a simple chocolate truffle; it is so simple that you’ll possibly not buy one ever again. You have two basic ingredients: chocolate and milk.
When you’re a cute little Chinese girl, it seems that your parents and all their friends will feed you only cute little Chinese things… har gow (round Chinese shrimp dumplings), gai don jai (Chinese mini pancakes as my parents called it), little Chinese white buns filled with yellow custard, and basically anything else round and small. Seeing a little girl eating these things is akin to that great video of the slow loris eating a rice ball.
I of course didn’t pick up on this until I had a revelation much later in life–probably why as a kid I was so punky and eventually developed a shrimp allergy. It is simple: I was popping too much of this cute food! These shrimp dumplings are about the size of a golf ball and whenever my family went for dim sum they’d put an entire tray of four in front of me at a time, which is prohibited in typical dim sum etiquette!
The mini Chinese pancakes are just that–pancake batter poured into a special pan that yields tiny, airy, crusty nubs of “cake”. It’s closest Western cousin is probably the cross between a pancake and less sugary, less dense madeleine. You have to eat them hot and fresh, and unfortunately they do not travel well. I’ve only ever seen them in street-carts and on the ends of ready-made food markets like the ends of Deluxe Supermarket in NY’s Chinatown. They are around $1 for 15 or 20, but when I was a kid they sold them by the whole sheets and wouldn’t break them up!