In a 10-inch nonstick skillet, whisk the condensed milk, cocoa, espresso, cinnamon and salt. Add the butter then set over medium and cook, stirring constantly with a silicone spatula and scraping along the sides, until the mixture reaches 225°F, about 10 minutes; it will appear matte and slide around freely in the pan. Immediately pour into a wide, shallow baking dish. Let cool to room temperature, about 30 minutes.