Mochi (餅) is Japanese rice cake made of mochigome (もち米), a short-grain japonica glutinous rice. The rice is pounded into paste and molded into the desired shape. Most common shapes are round and square. You can cook it by boiling or toasting over open flame or in a toaster oven/oven/broiler.
You can find Kirimochi (“cut” mochi) in Japanese grocery stores or Amazon.
Here’s the round kirimochi. Picture above is rectangular shape kirimochi.