How to Make It
Line an 8-inch square baking dish with foil, leaving 2 inches of overhang on 2 sides. Grease the foil with the softened butter.
In a medium heatproof bowl, combine the chopped chocolate with the sweetened condensed milk, cocoa, vanilla and kosher salt and place over—not in—a pot of simmering water. Cook, stirring occasionally, until the chocolate is completely melted and the mixture is smooth, about 15 minutes.
Scrape the fudge into the prepared pan and smooth the surface with a spatula. Sprinkle flaky salt over the surface while the fudge is still warm, so it adheres. Refrigerate the fudge until firm, about 2 hours.
Using the foil, lift the fudge out of the pan and cut it into sixty-four 1-inch squares.