Note: I use an 18x13 sheet cake pan.
In a mixing bowl, combine flour, sugar, and salt.
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 a measuring cup, pour the buttermilk and add beaten eggs, baking soda, and vanilla. Stir buttermilk mixture into butter/chocolate mixture. Pour into sheet cake pan and bake at 350-degrees for 20 minutes.
While cake is baking, make the icing. Melt butter in a saucepan. Add cocoa, stir to combine, then turn off heat. Add the milk, vanilla, and powdered sugar. Stir together and pour over warm cake.