16 tbsp. unsalted butter, cut in 16 pieces, plus 1 tbsp. softened butter for greasing pan
2-½ cups all-purpose flour, more for pan
2 cups granulated sugar
1 tsp. baking soda
¼ tsp. salt
¼ cup cocoa
½ cup buttermilk
2 eggs, lightly beaten
1 tsp. vanilla extract
For the Frosting:
8 tbsp. unsalted butter, cut in 8 pieces
¼ cup cocoa
6 tbsp. buttermilk
1 tsp. vanilla extract
¼ tsp. salt
1-pound confectioners' sugar
½ cup walnuts or pecans, chopped
Instructions
To prepare the cake:
Place oven rack in middle position; heat oven to 350°F.
Grease and flour a 9-inch by13-inch sheet pan.
Combine flour, sugar, baking soda and salt in a large bowl, and whisk well.
In a medium saucepan, over medium-low heat, combine butter, cocoa and 1-1/2 cups water; 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, place in pre-heated oven 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.
To prepare the frosting:
In a small saucepan, over medium-low heat, combine butter, cocoa and buttermilk; 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; mix until smooth.
Pour warm frosting over top of cake and smooth with a spatula.
Allow frosting to become firm before slicing cake.
Notes
Double (or triple) the cocoa in both the cake and frosting for a better result.