Preheat oven to 350 F degrees. Butter and flour a 9x13 inch baking pan.
In a small sauce pan, add the cocoa powder for the cake (1/4 cup), 1 cup of butter, and a cup of water. Bring to a boil, whisk and remove from heat. Set aside.
To a large bowl, add sugar, baking soda, salt, 2 cups flour and whisk. Make a well in the center and add the eggs, buttermilk and vanilla extract. Whisk or mix with a fork and gradually incorporate the dry ingredients. Add the cocoa/butter mixture and whisk until smooth. Pour batter into prepared baking dish and smooth the top. Lick the bowl.
Bake cake for about 35 minutes. Test doneness by inserting a toothpick in the middle, it should come out clean.
A few minutes before cake is done baking, make the frosting. In a small sauce pan, add butter, milk and cocoa powder and turn the heat to low. Whisk occasionally until butter is melted and mixture is smooth. Do not let boil. Remove from heat and whisk in the powdered sugar and vanilla.
Pour warm frosting over the warm cake and smooth it out. Top with pistachios and sea salt.
Serve warm or room temperature.
Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.