Pre-heat the oven to 350°F
Line the bottom of an 18 x 12 x 2-inch baking pan with parchment paper and spray generously with butter flavored cooking spray.
In the top of a double boiler, over medium heat, melt butter and 4 ounces chocolate; set aside.
In a large bowl, beat the granulated sugar and eggs until fluffy.
Add flour and salt; mix well to combine.
Add chocolate mixture; blending thoroughly.
Stir in the nuts and the vanilla.
Pour into the prepared baking pan.
Place in the pre-heated oven and bake for 35 to 40 minutes, or until top is crisp and toothpick inserted in center comes out clean.
Remove from the oven and cool conpletely before pouring frosting over the top.
