Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish. Sift together the flour, baking powder and salt; set aside.
In a large bowl, cream together 3/4 cup of butter, brown sugar and white sugar until light and fluffy. Beat in the eggs and yolk one at a time, then stir in the vanilla and 6 tablespoons peanut butter. Blend in the sifted ingredients and stir in the chocolate chips. Press the mixture evenly into the prepared pan.
Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Cool completely before frosting.
To make the frosting, mix the remaining 1/2 cup of butter and 1 cup peanut butter together in a medium bowl. Mix in the confectioners sugar until smooth, then gradually stir in milk until a spreadable consistency is reached. Cut, serve and enjoy!