Heat the cocoa powder, sugar and salt over medium low heat until the sugar has melted.
Stir in the milk, and raise the temperature to medium high.
Bring the mixture to a boil, and leave undisturbed until the temperature reaches 234 degrees as shown on a candy thermometer placed on the side of the pot.
Remove the pot from the heat, and add the vanilla and butter. Do not stir.
Wait until the temperature reads 130 degrees, and then stir for several minutes until the mixture begins to dull.
Pour into a well greased 9x13 baking dish, and drag a knife through the mixture to eliminate air bubbles.
Set for 2-3 hours, and then chill before serving.