This is Kashiwa mochi.
May 5th is Children's Day and a national holiday. The day was originally called Tango no Sekku and derived from China, but the contents has quite varied and the only similarity between Japan and China is to
eat Zongzi, sweet rice cakes wrapped in a bamboo leaf.
In Japan, we celebrate the growth of children, especially boys on the day. Families having a or more sons puts up Koi-Nobori, a carp streamer for the celebration. It was originally a Japanese custom that started among samurai families in the Edo period. And they also display Kabuto, a military helmet as a symbol of strength and vitality.
Kashiwa mochi is Japanese traditional sweet for the day. It is a rice cake with red bean paste inside wrapped in oak leaves, which are seen as a symbol of the prosperity of one's descendants.