Betty Crocker's recipe for peanut butter swirl brownies involves making two separate batters of peanut butter and chocolate and swirling them together before baking. The recipe takes about 50 minutes to prepare and makes 16 brownies. It is available at BettyCrocker.com.
Preheat an oven to 350 degrees Fahrenheit and grease a 9-by-9-by-2-inch square pan. In a mixing bowl, combine 2/3 cup of granulated sugar, 1/2 packed cup of brown sugar, 1/2 cup of softened butter, 2 tablespoons of milk and two eggs. Stir in 3/4 cup of all-purpose flour, 1/2 teaspoon of baking powder and 1/4 teaspoon of salt. Divide the batter into two separate bowls.
Stir 1/4 cup of peanut butter and 1/3 cup of peanut butter chips into one of the batters, and stir 1/3 cup of baking cocoa powder and 1/3 cup of semisweet chocolate chips into the other batter. Spoon the batters checkerboard-style into the pan, alternating the batters across 16 mounds. Use a knife to gently swirl through the batters, creating a marbled formation.
Set the pan in the oven, and bake for 30 to 35 minutes, until a toothpick inserted into the center comes out dry. Allow the brownies to cool completely for about one hour before slicing.