For the Frosting:
1/2 cup finely chopped pecans
1 3/4 sticks unsalted butter
4 Tablespoons (heaping) cocoa
6 Tablespoons milk
1 teaspoon pure vanilla extract
1 lb (minus 1/2 cup) powdered sugar
1 Tablespoon instant espresso coffee powder (optional)
In mixing bowl combine flour, sugar, salt. Set aside.
In a saucepan melt butter. Add cocoa. Stir together. Add boiling water, allow mixture to boil for 30 seconds, then turn off heat. Pour over flour mixture and stir lightly to cool. In measuring cup, pour the buttermilk and add beaten eggs, baking soda , and vanilla. Stir buttermilk mixture into butter/chocolate mixture. Pour into ungreased sheet cake pan and bake at 350-degrees for 20 minutes.
while the cake is baking, make the icing. Chop pecans finely. Melt butter in a saucepan. Add cocoa, stir to combine, then turn off heat. Add the milk, vanilla, powdered sugar and espresso powder if using. Add the pecans, stir together, and poor over warm cake.