Usually when you cook sugar to the softball stage you want it to stay nice and pliable and clear. But in the case of fudge you actually want to cook it to softball for the chewy-ness, and then agitate it by mixing, which starts re-inverting it back to crystalline form. It should be in very tiny crystals.
In a saucepan heat the chocolate and milk on low, stirring until it's melted. Add the sugar, corn syrup and salt and heat to the softball stage (236 degrees F).
Add the butter and pour off into a mixer bowl to cool. Add the vanilla and beat with a paddle until it starts to lighten; then quickly add the nuts. Continue beating on low until it loses its gloss and thickens. Pour into a greased 9 by 9-inch pan and spread evenly. Let cool then cut into 1-inch cubes.