How to turn a brownie mix into a cake?

My answer would be to add more cake flour if you have it, but AP flour will do it too, then add an additional teaspoon of baking soda.

The flour will lighten the mix so it's not so dense chocolate/cocoa, and the additional baking soda will help it rise more than a brownie.

An additional egg won't hurt, and it will certainly help the stickier of the cake, but I don't know if it's necessary.

Adding one more egg will give you more "cake like" brownies.

If you have baking soda you could use that as leavening. If it were me I would add 1/4 tsp. baking soda to the dry mix and stir to combine. I would add additional liquid in the form of 1/4 cup of buttermilk or soured milk to activate the soda and use an extra egg. This should give you something that is more cake like, than brownie like.

I made the brownies batter then decided I wanted a cake instead, so I added 1/2 of a yellow cake mix to the batter, plus 1 egg, 1/3 cup of vegetable oil and about 1/2 tsp of baking powder.
I mixed it all up and baked for 1 hour or so. I took way longer to cook, maybe because there was so much batter.

Pillsbury Brownie mix, Dark Fudge make cupcakes with crispy brownie top.
Mix It: Here is how to convert the brownie box mix to cake: Use extra large eggs, 33% more Canola oil, pretty warm Arabian coffee instead of water. Mix all liquid together first then add to brownie powder. Pan It: Spray cupcake liners with Bakers Joy. Put Oreo cookie in each cupcake well, 1/4 cup batter. I add 1/2 cup chopped walnuts to a box, BUT, add nuts by hand to each cupcake well, for even distribution. Bake It: Bake 325 F degrees for 40 minutes, toothpick comes out clean, because it's cake not brownies. Don't Worry: The cookies and nuts will float on top but it looks really kewl when done. :) Eat It: Dani"EL" Friend of God If I was making brownies then the toothpick comes out dirty with crumbs when done, BUT, cake toothpick comes out clean. I thought I messed up 3 boxes of brownies but I invented a new thing. My puppy "Summer Mousse" , like a dessert made me lose count of the oil. That's Some Moose .... :). I got about 4 1/2 dozen

Cinnamon PaZazzz Option: I also added 1 tsp of ground cinnamon to each box of brownie mix; I comment last because it matches the brownie frosting that goes with that option. Brownie Frosting: 6 tbsp butter softened, 2 1/2 cup confection sugar, 1 tsp cinnamon, 1 tbsp honey, 2 tsp vanilla extract, and make your own Dutch Chocolate ingredient: Heat up 1/3 cup of either: (heavy cream, half and half, evaporated milk or whole 2% milk) and add 3 1/2 ounce Unsweetened baking cocoa powder, a plus heaping 1/4 tsp baking soda) set aside.

Directions: Use stand mixer, whisk and cream room temperature butter; THEN, on low speed add in your Dutch Chocolate, cinnamon, honey, vanilla and confection sugar one cup at a time ( Might need to switch to beaters.). Continue to beat for 30 seconds.

When frosting just drop a tablespoon of frosting on top of each brownie; it will spread out by itself, do not spread it. (My brownies were cooling or about room temperature 80 degrees)

Add colorful sprinkles; placing in refrigerator will set the frosting.

So easy. Reduce fat by replacing some with unsweetened applesauce, add leavener (1 tsp baking powder), and possibly an egg or two. Good instructions here: https://www.bakingkneads.com/turn-brownie-mix-into-cake-mix/

Worked perfectly for me.