Head your oven to 350°F (175°C) and apply a non-stick spray to your baking tray.
In a spacious bowl, mix the melted butter with the sugar until it achieves a creamy consistency.
Introduce eggs sequentially, ensuring thorough mixing after each. Blend in the vanilla extract.
In another separate bowl, sift the flour, cocoa, baking powder, and salt together.
Gently incorporate the flour mixture into the butter mixture. If you fancy, fold in your choice of nuts.
Transfer the mixture to the prepared dish, and scatter chocolate chips uniformly on top.
Allow it to bake for 25-30 minutes. It's ready when a skewer comes out mostly clean from the center.
While the brownie bakes, prepare the frosting: In a saucepan, melt butter. Mix cocoa powder with milk and heat until it nears boiling.
Off the heat, blend in the powdered sugar and vanilla until velvety.
Once the brownie has cooled slightly, spread the frosting over the top. Let the baked treat sit on a rack to cool thoroughly before portioning.
For an enhanced serving, accompany a dollop of vanilla ice cream and a dusting of chopped walnuts or pecans, if you like.