The Collaborative International Dictionary of English v.0.48:

Brownie \Brown"ie\, n. [So called from its supposed tawny or
   swarthy color.]
   An imaginary good-natured spirit, who was supposed often to
   perform important services around the house by night, such as
   thrashing, churning, sweeping. [Scot.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

Brownie
    n 1: a junior Girl Scout
    2: (folklore) fairies that are somewhat mischievous [syn: elf,
       hob, gremlin, pixie, pixy, brownie, imp]
    3: square or bar of very rich chocolate cake usually with nuts