This is Japanese sweet rice ball, Ohagi. Ohagi is steamed sticky rice filled with sweet bean paste and covered with soybean flour. This is generally for spring equinox, but is sold anytime of the year in most of Japanese supermarket.
How does it taste? Imagine that, in a mashed potato, you fill with very sweet chunky peanut butter, and finish with a light dusting of cocoa powder.
Sounds yummy? I usually buy ones, but in case you want to cook it by yourself;