I use brownie mix in mini cupcake pans very frequently ... I add peanut butter cups, strips of Andes mints, chocolate kisses, Rollo caramels ... anything I have on hand.
But I bake much longer than 5 minutes.... more like 17 minutes. Maybe I'm using much more dough than you are??
I use the Chocolate Fudge cookie recipe and make my own Two bite brownies. Drop a large-marble size of mxiture into mini cupcake holders, bake about 5 minutes - they will puff then drop when taken out of oven. Every oven is different so you'll need to experiement. You dont' want to overbake them - they go hard. I can get about 72 two bite brownies from one cake mix.
