The most basic monaka come in a simple rounded-square shape, but there are also more elaborate ones that are made to look like flowers, blossoms, or chrysanthemums.
Their history goes back to the Edo period a few hundred years ago, when early iterations of the treat were invented. From there they evolved into a more recognizable sandwich cookie that resembles modern monaka around the Meiji Restoration in the mid-1800s. Now they continue to evolve; although the middle is still most commonly filled with red bean paste, some confectioners will put ice cream in the middle instead.
You can find monaka at Uwajimaya next to our other Japanese and Asian sweets.