For cake: Place oven rack in middle position; heat oven to 350 degrees. Grease and flour a 9-by-13-inch sheet pan. Combine flour, sugar, baking soda and salt in a large bowl, and whisk well.
Combine butter, cocoa and 1 1/2 cups water in a medium saucepan over medium-low heat; whisk frequently until butter has melted and mixture is smooth, glossy and bubbling around edges. Remove from heat.
Fold cocoa mixture into dry ingredients until just combined. Stir in buttermilk, eggs and vanilla until combined. Turn batter into prepared pan and bake until cake has risen, shrunken slightly from edges, and tests clean with a toothpick, about 25 to 30 minutes. Remove from oven, and cool completely on a rack.
For frosting: Combine butter, cocoa and buttermilk in a small saucepan over medium-low heat; whisk frequently until butter has melted and mixture is smooth and bubbling around edges. Remove from heat and transfer to a standing mixer bowl fitted with a paddle attachment.
With machine on low speed, add vanilla, salt, sugar and nuts, and mix until smooth. (Ingredients may also be whisked into cocoa mixture by hand; if so, sugar should be sifted first.) Pour warm frosting over top of cake, and smooth with a spatula. Allow frosting to become firm before slicing cake.
Mothers Secret Recipes of Comfort Food & Culinary Inspiration from an Organic and Allergen-free Approach