Heat oven to 350 degrees F. Coat an 8-inch cake pan with nonstick cooking spray. Line bottom with waxed or parchment paper; spray paper.
In small bowl, whisk together flour, salt and baking soda; set aside.
Microwave chocolate and butter in a medium-size microwave-safe bowl for 1 minute, stirring halfway through; microwave another 15 seconds if necessary and stir until smooth. Whisk in sugar and vanilla, then stir in eggs, buttermilk and 2/3 cup of the walnuts until well blended. Stir in flour mixture until just combined.
Pour batter into prepared pan. Bake at 350 degrees F. for 60 to 65 minutes or until a toothpick inserted in the center tests clean. Remove to a wire rack and cool in pan 5 minutes, then invert pan onto wire rack and cool completely. Remove and discard wax paper.
Place chocolate in small bowl. Heat cream in a saucepan over medium heat until just simmering. Pour over chocolate. Let stand 4 minutes to melt; stir until smooth.
Place cake on wire rack over baking sheet. Pour glaze over cake; use spatula to smooth over top and sides. Pat remaining 1/3 cup walnuts around top. When glaze is firm, remove cake to a platter and serve.