Sharad poornima | Amrut kheer | Milk payasam

Poornima/pournami happening in the month of Ashvin (Sep-Oct) is called Sharad poornima.

It is believed that moon rays contain Amrut (nectar). To take full effect of the amrut varsh by moon, people make kheer/payasam with milk (made by cooking rice in milk and adding sugar), and leave the milk under moonlight for whole night. Next morning, people take it as prasadam. 

https://www.drikpanchang.com/purnima/sharad/sharad-purnima-date-time.html