01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang.
02 - Whisk together melted butter and granulated sugar until well blended.
03 - Beat in eggs one at a time, then stir in vanilla extract.
04 - Sift cocoa powder, flour, salt, and baking powder into the wet mixture. Fold gently until just combined.
05 - Pour batter into prepared pan, smooth surface, and bake 25 to 30 minutes until a toothpick comes out with moist crumbs.
06 - Allow brownies to cool completely on a wire rack in the pan.
07 - Melt butter over low heat in a saucepan. Stir in cocoa powder until smooth.
08 - Whisk in milk and remove from heat. Gradually beat in sifted powdered sugar until thick and creamy.
09 - Stir in vanilla extract and a pinch of salt.
10 - Spread fudge frosting evenly over cooled brownies.
11 - Refrigerate for 15 to 20 minutes to set frosting before slicing.