Preheat oven to 180 degrees C. Grease a 20x30cm 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 centre comes out with moist crumbs. Cool completely before icing.
To make the icing, mix the remaining 1/2 cup of butter and 1 cup peanut butter together in a medium bowl. Mix in the icing sugar until smooth, then gradually stir in milk until a spreadable consistency is reached. Ice the brownies.