He also explains why Meemaw calls him by this nickname:
- "She calls me Moonpie, because I am nummy-nummy and she could just eat me up."
- —Sheldon during the same "episode"
When Wil Wheaton says he missed the Dixie-Trek convention because his grandmother had just died, Sheldon was stunned. He said that if anything happened to his Meemaw, he would be "one inconsolable Moonpie...". Stuart and Wil gave him quite a look, and he went on to explain that his Meemaw calls him Moonpie. 
"What up, Moonpie?"
-Penny's greeting to Sheldon in the same episode when coming back on the phone.
When Meemaw comes to visit in "The Meemaw Materialization", Amy learns about the nickname, which in turn, Meemaw comments how interesting it is that Leonard and Penny knew about the nickname while she didn't.
A large cookie-like pastry made from two round graham cracker cookies filled with marshmallow and dipped in chocolate.
Ah. Known in Great Britain, Canada (and seemingly Australia too) as a "Wagon-Wheel".