Do you mean a brownie that has peanut butter swirled into it?

This is my favorite brownie recipe: http://www.pbs.org/everydayfood/baking/recipes/mint_chocolate_brownies.html

If you leave out the mint, you can microwave about 1/2 cup of peanut butter (or more, according to your tastes) until it's fairly liquid, drizzle it over the unbaked batter, swirl it with a knife, and bake as directed.