These are fabulous, and very easy. It's hard to find a recipe that has both the chocolate and peanut butter layer both brownies - usually there's a frosting or glaze, but when you want the best of both worlds, this is the go-to recipe! From Gourmet magazine's December 1996 issue.
Preheat oven to 350 F. Grease and flour a 9-inch square pan, knocking out excess flour.
Break up unsweetened baking chocolate and melt in small bowl in the microwave in 30-second increments until it starts to melt, stirring after each increment, and then in 10 second increments until melted. (Note that once it's hot, it will continue to melt if stirred.) Let cool while continuing the recipe.
In a large bowl, beat (with electric mixer) the butter and sugar together until well-mixed, and then add the eggs, beating after each addition, and add the vanilla with the last egg.
In a small bowl, stir together the flour, baking powder and salt. Mix into the batter just until incorporated.
Evenly divide the batter between the two bowls. Stir the peanut butter into one bowl, and the melted chocolate and chips into the other.
Evenly spread the peanut butter mixture into the bottom of the prepared pan. Spoon the chocolate mixture over the peanut butter mixture and spread gently and evenly.
Bake 35 - 40 minutes, until a toothpick comes out clean or with dry crumbs (but note that with the melted chips, you may need to check in a few spots).