Adding a few more details to Explosion's description, since I just happened to read anko3577, "Save Her, Okay?"
It starts with the father Marisa begging passersby to save her little one. A young girl hears her say, "Marisa doesn't care what happens to herself, just save the little one!" and takes her at her word.
This looks like a typical abuse story that involves skinning and blinding a koyukkuri, chopping the top of the mother then having the half-dead senseless koyukkuri, which believes its being feed sweets to help it recover, eat the paste core out of its parent.
After this, the girl puts the top of the head back on and adds some orange juice, sealing Little Marisa inside. Little Marisa's bean paste merges with her father's bean paste around her, and when she wakes up, she's inhabiting and animating her father's body. It takes a while for the girl to convince her that the "Daddy" she sees is in the mirror actually "herself." No trace of father Marisa's consciousness seems to remain.
Fascinated by the results of this experiment, the girl decides to keep Marisa as a guinea pig so she can learn more about yukkuris.