Start by making the peanut butter mix that will be added to the brownies. In a small bowl add 1/3 packed brown sugar, 1/3 crunchy peanut butter, and 1 egg. About the actual peanut butter--we only had smooth peanut butter (remember people in the house don't like nuts), so use what you have. In hindsight the nuts might help cut down on the richness but it's your call.
Make the brownie mix according to the box.
Preheat the oven depending on what pan you bake this in.
Here comes the fun part. Put 3/4 of the brownie batter into your pan.
Here's a secret: it's sometimes really hard to cut brownies or bars. So you can use a knife or a plastic knife and gently saw. Or you can use a pizza cutter. Yup a pizza cutter. If you get any brownie on the blade just wipe it off, but it leaves pretty clean lines. So easy now. Do you have a good way to cut brownies?
About a tablespoon at a time (give or take a bit) drop the peanut butter mixture into brownie mixture. Spread it out so that you have peanut butter throughout the pan.
Then cover the peanut butter with the remaining brownie mixture left over.
Take a knife and gently sweep it back and forth mixing the peanut butter and brownie batters. It's not an exact science you just want to mix the two.
Bake according to the box.
While the brownies are baking it's time to make the frosting. Take a container of vanilla frosting and scoop 3/4 into a bowl. Then add 1/4 peanut butter (you probably want smooth but it's up to you) and then 2 teaspoons milk.
Combine the ingredients together and there's the frosting. Now just wait for the the brownies to cool and then add on the frosting.
Recipe (adapted from Betty Crocker)
1 box brownie mix
1/3 cup packed brown sugar
1/3 cup crunchy peanut butter
2/3 cup vanilla frosting (from 16 0z container)
1/4 peanut butter
2 teaspoon milk
Combine brown sugar, peanut butter and egg in small bowl.
In separate bowl make brownie mix according to package.
Put 2/3 of brownie mix into 13 x 9 pan.
Scoop tablespoons of peanut butter mixture into brownie mix
Cover peanut butter with remaining brownie batter
Bake per box
Make frosting by combining vanilla frosting, peanut butter and milk in bowl.
Allow brownies to cool on a wire rack.
Frost brownies and have milk ready to drink!