In a medium saucepan, stir together the cocoa powder, sugar and salt. Mix in corn syrup, and milk until well blended. Add butter, and heat to between 234 and 240 degrees F (112 to 116 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface. Stir occasionally.
Remove from heat, and beat with a wooden spoon until the mixture is thick and loses its gloss. Stir in vanilla, and pour into a buttered 9x9 inch baking dish. Let cool until set. Cut into small squares to serve.