Preheat oven to 325 F. Line a 9 X 13-inch pan with foil so that the foil hangs over the edge of the pan. Generously mist with cooking spray. Set aside.
In a microwave safe bowl, combine butter and unsweetened chocolate and cook for one minute. Stir until smooth (an additional 30 seconds if needed). Transfer to a large mixing bowl. Add oil, sugar, brown sugar, and beer. Mix until smooth. Add eggs, yolks, and vanilla. Mix until smooth.
In a separate bowl combine flour, cocoa powder, salt and Clabber Girl Baking Powder. Gradually add to liquid ingredients using a wooden spoon. Stir until just combined. Fold in chocolate chunks and pistachios.
Bake for 35 to 45 minutes or until a toothpick comes out with a few fudgy crumbs when inserted into the center. Cool completely.
Irish Stout Buttercream Preparation: With a mixer cream together powdered sugar, butter and stout beer until light and fluffy. Using the foil, pick up the brownies and remove them from the pan. Spread buttercream frosting over the top of the brownies. Cut and serve.