Green’s Dictionary of Slang

(US black/W.I.) a little extra, like the thirteenth biscuit in a baker’s dozen, or an extra helping of food; in musical shows it has come to be the encore.

1912		C. McKay Songs of Jamaica 38: No two bit o’ brater / Wid shopkeeper Marter.
1926		in J.F. Dobie Rainbow in Morning (1965) 57: When a negro mother whips her child she gives it an extra lick for ‘brawtus’.
1930		G.B. Johnson ‘Speech of the Negro’ in Botkin Folk-Say 356: Brawtus, meaning something thrown in for good measure, was recently cited by Professor Robert A. Law [...] as probably an African word.
1943		cited in Cassidy & LePage Dict. Jam. Eng. (1980) 64/2: Brawta, Brata an addition, makeup.
1981		N. Farki Countryman Karl Black 85: He had got his pay with a lot of ‘brawta’.
1995		Francis-Jackson Official Dancehall Dict. 6: Brawta extras; surplus: u. beg yuh a brawta/may I have some extras.