In what country do people put out a bowl of porridge as a Christmas treat for the barn elf?

denmark

Scandinavia in general -- A tomte or nisse (brownie) is a mythical creature of Scandinavian folklore, specifically part of a group of creatures called vetter (elf), common in rural areas. There were two varieties of nisse or tomte, one type that resided in the house, and one that resided in the barn. He would take care of the homestead, crops, house, and people while the human inhabitants slept.

Like the brownie, the tomte or nisse needed gifts, but this gift was a bowl of porridge or rice pudding on Christmas night. If he wasn't given his payment, he would leave the farm or house which couldn't survive without his nightly chores, or engage in mischief, such as tying the cows' tails together in the barn, turning objects upside-down, and breaking things.

http://encyclopedia.thefreedictionary.com/Tomte

Denmark -- A Danish Tomten was my choice. On a trip to Denmark I learned that a Tomten is a Danish elf who lives secretly in the barn. If the farmer offers him porridge, the Tomten helps things along on the farm. But if the farmer ignores the little elf, things go mysteriously wrong!

http://www.barnesandnoble.com/w/hedgies-surprise-jan-brett/1103362842

Norway -- Christmas Eve dinner begins with rice porridge which contains one lucky almond. A bowl is also set out for the barn elf so that he will continue to watch over the animals and not turn mischievous

http://christmas-world.freeservers.com/norway.html