It was in Bunbunmaru this mroing. A vampire in SDM who had eat her six yukkuri. I am truly sorry for their lots.
Seriously though, the whole "yukkuri" thing came about after somebody posted a truly terrible attempt at ASCII fanart of Reimu and Marisa. Basically they looked like a pair of disembodied bean heads. The rest, as they say, is history.
For more information on how the Yukkuri phenomenon started, read the Yukkuri bible.