Postion the oven rack in the center of the oven. Heat the oven to 350 F and spray a sheet pan with baking spray.

In a saucepan, combine the water, 1 cup of butter, 3 tablespoons of cocoa powder; stir to blend. Place the pan over medium heat and bring the mixture to a boil.

While the cocoa mixture is heating, combine the flour, granulated sugar, baking soda, and 3/4 teaspoon of fine sea salt in a large bowl.

In a medium bowl, whisk the buttermilk with the eggs and 1 1/2 teaspoon of vanilla.

Add the buttermilk mixture to the dry mixture and stir until blended.

Carefully add the boiling cocoa mixture and whisk until smooth and well blended.

Spread the batter in the prepared sheet pan and bake for 16 to 20 minutes, until the cake bounces back when lightly touched with a finger.

Meanwhile, prepare the icing. Combine the milk in a saucepan with 1/4 cup of cocoa, 1/2 cup of butter, and a dash of salt; stir to blend. A few minutes before the cake is finished baking, bring the icing mixture to a boil. Whisk in the vanilla and powdered sugar, then fold in the walnuts or pecans.

Pour the hot icing over the hot cake and quickly spread it to cover the entire cake.

Let the cake cool for at least 1 hour before slicing.