Combine dry ingredients in large bowl; sugar, flour, cocoa, baking powder, baking soda, and salt. Add eggs, buttermilk, oil and vanilla; beat on medium speed for 2 minutes.
Stir in boiling water; batter will be thin. Pour into baking dish.
Bake 25 to 35 minutes or until wooden toothpick inserted in center comes out clean.
Cool cake for 15 minutes on wire rack.
Frost with Chocolate Walnut Fudge Frosting while cake is warm but not hot.
Make Chocolate Walnut Frosting
Melt together butter and cocoa. Add milk and vanilla. Stir in powdered sugar and nuts.