Preheat oven to 350 degrees F. Line a 13×9 inch metal pan with foil and spray bottom with cooking spray or rub with butter.Cream the butter and sugar with an electric mixer. When light and fluffy, add peanut butter and beat until incorporated. Beat in eggs, the egg yolk and vanilla. By hand, stir in the flour and chocolate chips.
Spread batter in pan, patting it down flat, and bake on center rack for 35-40 minutes or until puffed and golden. Cool completely.Make the ganache. **
Put the chocolate chips in a heat-proof bowl. In a saucepan or in a microwave-safe measuring cup, heat the cream until starts to boil.
Pour the cream over the chips and let stand for a minute. Stir well to melt chips; stir in the butter.
Spread on cooled brownies and let set.Makes 36 bars
