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1
For cake: Place oven rack in middle position; heat oven to 350 degrees.
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2
Grease and flour a 9-by-13-inch sheet pan.
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3
Combine flour, sugar, baking soda and salt in a large bowl, and whisk well.
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4
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.
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5
Remove from heat.
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6
Pour cocoa mixture over dry ingredients and whisk until smooth.
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7
Whisk in buttermilk, eggs and vanilla.
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8
Turn batter into prepared pan and bake until cake has risen, shrunken slightly from edges, and tests clean with a toothpick, about 30 minutes.
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9
Remove from oven, and cool completely on a rack.
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10
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.
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11
Remove from heat and transfer to a standing mixer bowl fitted with a paddle attachment.
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12
With machine on low speed, add vanilla, salt, sugar and nuts, and mix until smooth.
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13
(Ingredients may also be whisked into cocoa mixture by hand; if so, sugar should be sifted first.)
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14
Pour warm frosting over top of cake, and smooth with a spatula.
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15
Allow frosting to become firm before slicing cake.
