Whenever someone had a birthday, these little buns filled with either lotus seed paste or red bean paste would appear. But why? The reason was because they resembled the Peaches Of Immortality.
In Chinese mythology, Peaches of Immortality are consumed by the immortalsdue to their mystic virtue of conferring longevity on all who eat them. Peaches symbolizing immortality (or the wish for a long and healthy life) are a common symbol in Chinese art, appearing in depictions or descriptions in a number of fables, paintings, and other forms of art, often in association with thematically similar iconography, such as certain deities or immortals or other symbols of longevity, such as deer or cranes.— Wikipedia
According to legend, the deities of heavens feasted on these peaches to maintain their immortality. Hence, these were rare and very valuable as they took thousands of years to ripen.