WoWDB

 Romantic Picnic Basket are sold by Lovely Merchants during the  Love is in the Air event.

 Buttermilk Delight chocolates are found randomly in  Box of Chocolates, also sold by Lovely Merchants.

You do not need to own a Romantic Picnic Basket of your own, you just need to sit at one (with a companion) and eat the chocolate in Dalaran.

Also, although chocolates cannot be sold in the Auction House or mailed, they can be traded between players.