A monaka is a Japanese “cookie”, belonging to the class of Japanese desserts known as “Wagashi.”
It consists of 
crisp wafers made from a sticky rice dough, with adzuki bean filling (called “anko”) between them.
The filling can also be be made from sesame seed or chestnut purée. Modern versions also put ice cream between the wafers.
The shape is usually round, but can be square or triangular.
The wafers are imprinted with a flower pattern, such as cherry blossoms, chrysanthemum, etc.
Monaka are usually served with tea.