in a double boiler, melt butter and 4oz of chocolate. set aside.
In a large bowl, beat sugar and eggs until fluffy. Add the flour and the salt, mixing well. Add chocolate mixture, blending thoroughly. Stir in the nuts and vanilla.
Pour into prepared pan. The batter should be about 1/2 inch thick , no more. Bake for 35 to 40 minutes or until top is crisp and tooth pick inserted comes out clean. Allow to cool before applying the frosting.
To prepare the frosting:
Bring cream to a boil, stirring constantly. Place the chopped chocolate into a bowl and pour the boiling cream over the chocolate and stir to blend well. Spread a thin layer of warm frosting over the top of the brownies in the pan. When cool, cut into 2 inch squares.