This is a link to the last time I talked about this here:

https://news.ycombinator.com/item?id=16534745

and to the particular comment in that subthread where I give the recipe itself:

https://news.ycombinator.com/item?id=16540539

EDIT: actually, let me also just paste the recipe here:

    3¾c. of brownie mix (again, Betty Crocker Original Supreme)
    2 eggs
    ¼c. water
    ⅓c. vegetable oil
    and the included packet of Hershey's syrup

The baking times on the old box were: for 13x9 pan, 28-30m at 350°, for 9x9 pan, 35-40m at 350°, and for 8x8 pan, 50-55m at 325°. (We usually use the 13x9, can't speak as much to the other sizes.)

As of the original redesign mentioned in [the 2018 article I'm pasting this from], the amount of brownie mix in a box was cut back to 3 cups, the recipe involved 1 egg instead of 2, and I don't remember how the water and oil were affected but they were different.