A Parent's Love
This sidequest is unlocked once you have advanced and almost finished the sidequest "From the Ground Up" to build Tarrey Town. Learn more about it here.
At night, go to the house where a mother is tending to her sick daughter. She won't tell you outright what's happening, so eavesdrop from outside the window in the back of the house. It seems that the little girl will only eat one specific type of cake.
The cake in question is hinted at by how desperate the mother sounds. You need to cook a Monster Cake. The ingredients are: Tabantha Wheat, Goat Butter, Cane Sugar, and Monster Extract. You can buy the first three ingredients from general stores around Hyrule, but Monster Extract is special.
If you haven't yet, an NPC at the Dueling Peaks Stable will gift you with one and hint about Skull Lake. If you have not unlocked Kilton yet, you can do so by completing the sidequest "A Shady Customer". Learn about it here. Exchange monster parts for mon to buy the Monster Elixir.
The other three ingredients can all be bought from the general store at Rito Village. Then you can use the cooking pot provided for convenience before returning to Tarrey Town.
Once you have cooked the cake, bring it to the family. You will be warped back outside, so go back inside and speak with the mother, Ruli. She will give you a gold Rupee worth 300 Rupees as thanks for helping her daughter recover.