I'm not 100% sure which brand I've been using, but I do the dark chocolate brownie mix that's boxed ( I cheat :oP), and add 1/2 cup melted peanut butter to the batter and mix well. Also, I use 1 more egg than what the recipe calls for (as stated on the box: for more cakelike brownies).
Pour into an 8x8 or a 9x9 pan. Heat another 1/2 cup peanut butter and drizzle on top of the batter before putting into the oven. Don't mix in or anything.
Bake as directed on the box, and voila! Delicious, peanut butter brownies that are thick and yummie. The dark chocolate makes it not as sweet as the regular chocolate with the peanut butter.