The term I would use for the leftover batter from a mixing bowl used for brownies or banana bread is scrapings. To my surprise, Merriam-Webster's Eleventh Collegiate Dictionary (2003) doesn't have an entry for scraping or scrapings as a noun, although a Google Books search finds instances of the plural form (in the sense of "things scraped off") going back to 1651, and hundreds of matches during the period from 1650 to 2000.
Here are some instances of the word in use in this sense. From E.B. Bennion, A.J. Bent & G.S.T. Bamford, The Technology of Cake Making (1997):
Scrapings from the bowl should, if possible, be kept for the next batter. but if it is necessary to use them they must be mixed into the batter. Otherwise, if placed on top they show up in the crumbs of the cake as dark streaks.
This recommendation echoes advice given more than a century earlier in Mrs. [Mary] Lincoln's Boston Cook Book (1884, revised 1900):
Put the scrapings from the bowl into small tins. If put into the loaf, they sometimes cause a heavy streak through the top. Cake is baked when it shrinks from the pan and stops hissing, or when a straw inserted in the centre comes out clean.
Likewise, from American Cookery (1926) [wording doesn't appear in snippet window]:
Just the scrapings from the dish in which oatmeal was made for breakfast makes very good thickening for a mixed vegetable soup.
The term also appears in fictional writing. From Rowena Summers, Daisy's War (2001) [snippet]:
'Can I lick the spoon?' he asked hopefully, knowing he wouldn't have to share the delicious scrapings in the bowl with anyone else now.
'Of course you can, darling. It'll be all yours.'
From Beth Harlow, The Journal, Lost Memoirs from the Civil War (2013):
Today James and I had a treat. We went into the kitchen of a house when the cook had just put the cornbread in the stove and asked if she had any food she could spare. She let us have the scrapings in the bowl where she had mixed the cornbread. We shook up the scrapings with water, put our bacon in and boiled it to make a soup. It was delicious.