I don't. But what's your goal? A brownie batter would probably contain some ingredients you don't want in beer.
I was just thinking a brownie stout would be nice. Something as close to a brownie taste as possible. I've never really had a stout where I thought man that tastes like brownies. And have seen that Weldwerks did one, but not had it.
The page here says it is a: "Milk Stout brewed with lactose, cocoa powder, cacao nibs, milk chocolate, dark chocolate, vanilla and of course, brownie mix." Honestly once you get through all of those ingredients, I'm not sure why you would still add brownie mix, as I thought brownie mix was more or less a combination of those ingredients in the first place (with eggs, butter/oil added). Surprised by the milk/dark chocolate as well because of the fat content. Oh and to answer your question, adding the powder/nibs/vanilla after primary is complete is fairly standard. I guess brownie mix could be the same if you really still want to use it (though you can expect it to kick fermentation back off with the sugars it contains). Lactose can go in the boil.
I would avoid adding any fats/oils other than the ones unavoidably embedded in any cocoa powder/nibs you might use. I think I'd give eggs a miss too.
Yeah, it's as if "first make some brownie mix from scratch, then run to the store and buy some brownie mix and use it too." FWIW, Duncan Hines Milk Chocolate Brownie Mix ingredients: "Sugar, Enriched Bleached Wheat Flour (Flour, Niacin, Reduced Iron, Thiamine Mononitrate, Riboflavin, Folic Acid), Vegetable Shortening (Palm Oil And/Or Partially Hydrogenated Soybean Oil), Cocoa Processed With Alkali. Contains 2% Or Less Of: Dextrose, Salt, Wheat Starch, Natural And Artificial Flavors, Acacia And Xanthan Gums, Leavening (Sodium Bicarbonate)." I'm unconvinced of the goodness to be had with additions of Xantgan Gums or Artificial flavors. By comparison Annie's Organic Double Chocolate Brownie Mix sounds more benign but still has the added oil of cocoa butter.
In addition to avoiding eggs and oil, the flour in brownie mix would not be all that helpful for your beer. Unfermentable starch? Unless you are shooting for New England Brownie IPA, I suggest you pass.
While no where near an expert, this is exactly why I got a three gallon carboy. Do a test batch and hate, so chuck it. I would say brew a milk stout and cocoa powder the hell out of it with some vanilla beans and then see what needs to be added. I had the same thoughts about all the flour. One hell of a thick stout if that makes it in there.
I lean this way. And if you don't like the ratios, cut yourself a bigger brownie or get yourself a bigger stout.
Actually egg whites are a pretty common ingredient in wine...used as a fining agent to remove excessive tannins from an overly astringent wine. So you could definitely add that..but it should not be enough to affect flavor. Otherwise agree with above comments that I'd skip the other stuff and just focus on the deconstructed ingredients such as lactose, cocoa nibs, vanilla, salt and maybe some cocoa powder. If you desire to add the brownie mix, add it to the top of your mash at the beginning of the mash and be sure to cut the grain bed surface several times if fly sparging.
Or.... Get ready for this... Pulverize the brownie in a food processor, then mix it into a stout like a smoothie.
LOL... I wonder if that has anything to do the crap load of oil and fats they added to it via chocolate and brownie mix.
You could also French press a brownie into a stout. FO seems to suggest this flavor, although I think it has more to do with how BL brews it rather than adding any brownie "adjuncts."