01 - Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy brownie removal.
02 - In a large bowl, whisk melted butter and granulated sugar until well combined.
03 - Beat in eggs one at a time, followed by stirring in vanilla extract.
04 - Sift cocoa powder, flour, salt, and baking powder into the wet mixture. Stir gently until just combined without overmixing.
05 - Pour batter into the prepared pan and smooth the surface evenly. Bake for 25 to 30 minutes, or until a toothpick inserted near the center comes out with moist crumbs.
06 - Allow the brownies to cool completely in the pan placed on a wire rack.
07 - In a medium saucepan over low heat, melt butter and stir in cocoa powder until smooth.
08 - Whisk in milk and remove from heat. Gradually incorporate powdered sugar, beating until the frosting becomes thick and creamy.
09 - Stir vanilla extract and a pinch of salt into the frosting mixture.
10 - Spread the prepared fudge frosting evenly over the completely cooled brownies.
11 - Refrigerate the frosted brownies for 15 to 20 minutes to set the frosting before slicing for clean edges.