In a medium heavy-bottomed pot, whisk together sugar, cocoa powder, and salt until fully combined. Switch to a wooden spoon, stir in milk gradually, and add the butter.

Set the pot over medium heat. Stir constantly and bring it to a full, rolling boil that can’t be stirred down. Let it boil for exactly 3 minutes and 45 seconds.

Take the pot off the heat. Stir in the vanilla extract. Let it cool for no more than 5 minutes—it should still be warm and pourable.

Using a hand or stand mixer with a whisk attachment, start on low speed and gradually increase to high. Beat for 5 to 10 minutes, until the icing thickens, becomes creamy, and loses its shine. Watch the edges of the bowl for stiffness—this means it’s ready.

Pour and spread the icing over cake or brownies while still warm. Add chopped nuts on top, if desired.