"Hash brownies” are “brownies” (the familiar chocolate treat) made with “hash” (cannabis). The name “hash brownie” is a pun from the food called “hash browns,” although the “hash brownies” do not contain potatoes.
The Hashish Cookbook (1966) contained a recipe for “hash brownies” and helped to popularize the food (illegal in many places in the world). Another name for “hash brownie” is “space cake.”
Wikipedia: Cannabis foods
Cannabis foods concerns the preparation and cooking of foods with the drug cannabis in herbal or resin form as an alternate way to experience the effects of the drug without smoking it. Commonly it is cooked into a cake, cookie, brownie, or other baked product to be distributed socially. There are many different names and slang terms for these recipes. Most are based on standard recipes for brownies, cakes or cookies. Often, prefixes such as hash, cannabis, weed, pot, space, cosmic, magic, special, enhanced are added to the name of the food that they are prepared with: “hash cakes,” “special brownies” etc.
(Oxford English Dictionary)
hash brownie n. [punningly after hash browns n. at HASH n.1 Compounds] slang (orig. U.S.) a brownie or other cake containing cannabis, eaten as an intoxicant.
1966 ‘PANAMA ROSE’ Hashish Cookbk. 15 *Hash brownies… Pulverize 5 grams of top quality hash. Melt over hot water: 2 oz. unsweetened chocolate [etc.].
1979 Syracuse (N.Y.) Herald Jrnl. 7 Dec. 18/5 Rae unwittingly got high on Kim’s hash brownies and was hospitalized after walking into traffic.
1993 Guardian 18 Sept. (Weekend Suppl.) 21/4 Mike Goodman insists more people now roll joints and eat hash brownies than attend football matches, art galleries or church.
OCLC WorldCat record
The hashish cookbook
by Panama Rose.
Type: Book; English
Publisher: [s.l.] : Gnaoua Press, 1966.
16 August 1967, New York (NY) Times, “Bells, Books and Candles: Psychedelia on Sale” by Angela Taylor, pg. 44:
...and the Hashish Cook Book ($1.65). The book contains recipes for Hash Brownies, and Betel Nut Malteds, but Miss Lemish pointed out that some of the basic ingredients are unobtainable.
17 September 1969, New York (NY) Times,"Hashish Fad in U.S. Worries Officials” by Lesley Oelsner, pg. 32:
If you were to eat hash brownies, for instance, you have more likelihood of having a bad trip.
25 August 1975, New York magazine, “What Will Happen When Middle-Class America Gets the Straight Dope?” by Albert Goldman, pg. 28, col. 3:
The craziness it can induce when smoked is magnified even further when it is ingested in hash brownies or other pastries.