In a medium heavy-bottomed saucepan, melt the butter over medium heat. Stir in the sifted Dutch-process cocoa powder and whole milk. Bring the mixture to a gentle boil for 1 minute, whisking constantly until smooth and glossy (this blooms the cocoa for maximum rich chocolate flavor).

Remove the saucepan from the heat. Pour the warm chocolate mixture into a large mixing bowl. Add the pure vanilla extract and sea salt.

Gradually add the sifted powdered sugar 1 cup at a time, beating on medium speed with an electric hand mixer (or vigorous whisk) until the frosting is thick, silky, and glossy. If too thick, beat in 1 tablespoon of warm milk until smooth and spreadable.

Use immediately to spread over cooled yellow cake, chocolate cake, brownies, or cupcakes with an offset spatula. The frosting will set into a velvety, sliceable fudge crust within 20 minutes at room temperature.