FOR BROWNIES

1. Preheat oven to 350 degrees (180C). Spray a 13×9-inch baking dish with nonstick spray. Line the baking pan with parchment paper and spray the paper lightly again.
2. In a bowl, beat together the butter and sugar until light and fluffy. Scrape down the sides to ensure it is all evenly whipped.
3. Beat in one egg at a time into the creamed butter and sugar, scraping down the bowl as necessary. Beat in the vanilla extract. Beat mixture until light and fluffy.
4. In a separate bowl, whisk together the flour, cocoa, baking powder, and salt until combined. Slowly add into the whipped butter mixture until just combined. Lastly, stir in the chopped walnuts.
5. Spread the batter into the prepared pan. Bake on the middle rack, for approximately 25-30 minutes until a toothpick inserted in the middle comes out clean. Allow the brownies to cool completely.

FOR THE FROSTING

1. In a bowl, beat the butter until smooth. Beat in the powdered sugar, cocoa powder, and milk until the frosting is smooth. Scrape down the bowl as needed until the frosting is smooth, and fluffy.
2. Spread evenly over the brownies. Slice and serve.