Combine the dry ingredients like flour, sugar, baking soda and salt in one bowl and wet ones like buttermilk, eggs and vanilla in another.
Next, melt the butter in a saucepan. Add the water and whisk in the cocoa powder and bring to a boil. Remove from heat.
Add the cocoa mixture to the dry ingredients and stir or use electric beater to combine. Add the wet ingredients and mix to combine into a smooth batter.
Pour the batter into a parchment paper lined 13x18 inch sheet pan (yes, this makes 24 servings). Bake in a 350°F oven for 20 minutes.
Frosting
Make the frosting by combining the butter, cocoa, and buttermilk in a saucepan and bring to a boil. Remove from heat and stir in the vanilla, salt and powdered sugar until smooth.
After cake has baked, DON'T COOL, but pour warm frosting on top and spread evenly. Sprinkle chopped pecans on top. Allow to rest on wire rack until set.
Notes
All you need to do is pour just under 1/2 cup regular milk (not low fat) into a measuring cup and add enough regular vinegar or lemon juice to the milk to bring the quantity up to 1/2 cup. It will work it's magic in seconds after stirring and you're good to go in a pinch.
You can substitute regular cocoa, but the dark is richer. Also, you could add a teaspoon of espresso powder as well to make the chocolate richer in taste.
This is key as it creates a super moist, gooey layer between the cake and the frosting.