Green’s Dictionary of Slang

(US drugs) amphetamine.

1969		R.R. Lingeman Drugs from A to Z (1970) 52: brownies (1) [...] amphetamines; perhaps specifically, dexedrine Spansule capsules. (2) [...] durophet-m capsules.
1981		D.E. Miller Bk of Jargon 337: brownies: Probably Dexedrine, an amphetamine.
2001		ONDCP Street Terms 4: Brownies — Amphetamine; Browns — Amphetamine.