Preheat oven to 350°F. Grease a 9x13 pan.
Whisk melted butter and cocoa until smooth.
Stir in sugar, flour, and salt until thick.
Add eggs one at a time, then vanilla; mix until glossy.
Spread into pan and bake 20–25 minutes (do not overbake; toothpick should have moist crumbs).
In last 5 minutes, whisk frosting ingredients until smooth and pourable.
Remove brownies and immediately pour frosting over hot brownies; spread quickly.
Cool 30–45 minutes until frosting sets into a thin crackly layer.
Tip: If frosting is too thick, add milk 1 tsp at a time. If too thin, add powdered sugar 1 tbsp at a time.