In a medium saucepan, melt the butter over low heat. Stir in cocoa powder. Once the mixture has melted and comes to a light simmer, remove from heat. Slowly whisk in milk, vanilla, and salt. Then slowly stir in powdered sugar.
Pour the icing over your sheet-pan cake or brownies the moment they come out of the oven. Spread evenly with a rubber spatula immediately.
Notes
Nut lovers: Chop 1/2 to 3/4 cup pecans and mix into the frosting. Halve the recipe: This recipe will cover a cake or brownies the size of a half-sheet baking pan (perfect for Texas sheet cake). You can halve this recipe for a 9x13-inch cake or brownies.