brownie

1. a small square of rich chocolate cake, typically containing nuts.
2. a member of the junior branch of the Guide Association, for girls aged between about 7 and 10, wearing a brown uniform.

   for my birthday I want to join the Brownies

3. a benevolent elf that supposedly haunts houses and does housework secretly.

early 16th century (in brownie (sense 3)): diminutive of brown (a ‘wee brown man’ often appears in Scottish ballads and fairy tales).