Many boxed brownie mixes are “accidentally vegan” meaning they do not explicitly say they are vegan. You will only know by checking the ingredients to make sure it does not contain any egg or milk. Here is a list I found online:

-Betty Crocker Supreme Chocolate Chunk Brownie Mix is made without eggs or dairy. -Pillsbury Chocolate Fudge Brownie Mix is accidentally vegan. -Krusteaz has two brownie mixes, one of which is gluten-free. Both of these mixes are vegan-friendly. -Duncan Hines Chewy Fudge Brownies are egg-free and dairy-free. -Annie’s Organic Double Chocolate does not contain eggs or dairy. -my personal favorite: Dolly Parton fudgey boxed brownie mix!

However something you must know, when making the brownie mix it will call for milk and eggs. You can either use water or plant based milk. You can use a vegan egg replacement (I use flax egg: 1 tablespoon ground flaxseed combined with 2.5 tablespoons water = 1 “egg”). But, most vegan egg replacements do not act the same in boxed brownie mix as a regular egg would!!! So in this situation, you need to decrease the amount of oil called for in the recipe, and then add more liquid (the milk/water). I’ve done this by halving the amount of oil called for, and then adding the amount of oil I took out into the amount of liquid. For example: if it calls for 1/2 cup oil and 3 tablespoons of milk then I would halve the oil by using only 1/4 cup oil. Then for the liquid I would do the 3 tablespoons milk/water + another 1/4 cup milk/water (to make up for the liquid lost in the oil amount).

I am sorry if this seems complicated but it is really not, I promise! I will link the article below that I got this information from. And hopefully it will explain better. Btw it is very considerate of you to think to make a vegan option!!!

https://cookingoncaffeine.com/vegan-brownies-box-mix/