Preheat oven to 350 degrees. Line a 9x13-inch baking dish with foil and spray foil with non-stick spray. Set aside.
CAKE: In the bowl of an electric stand-mixer, using the paddle attachment, add the melted butter and cocoa powder, blend until smooth.
Add the flour and sugar, mix until combine, then add eggs and vanilla. Mix just until combined. Don't over mix. (Batter will be thick) Stir in nuts if using. Pour batter into prepared baking dish and spread out evenly.
Bake 25 to 30 minutes or until toothpick inserted in center comes out clean. Remove brownies from oven and let stand 15 minutes before frosting. You want the brownies still warm when you frost them!
CHOCOLATE FROSTING: In a large bowl, beat together all of the frosting ingredients until smooth. Spread over warm brownies. Let cool completely before cutting and serving. Makes 12 large or 24 small brownies