ounces (4 cups) powdered sugar, or as needed to reach desired consistency
cup finely chopped pecans or walnuts, optional
For the Cake:
Preheat oven to 350 degrees F. Coat a 13" x 9" baking dish with non-stick cooking spray.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt. Using an electric hand or stand mixer, beat in eggs, buttermilk, oil, and vanilla. Add hot water and mix until combined. Batter will be quite thin.
Pour batter into prepared baking dish. Bake for 35 to 40 minutes or until a toothpick inserted in center comes out clean. Cool completely in pan before frosting.
For the Frosting:
Melt butter in a large saucepan over MEDIUM heat. Whisk in cocoa powder, buttermilk and vanilla. Turn heat to LOW and whisk in powdered sugar until you've reached desired consistency. It should be thick but pourable. Continue whisking vigorously until smooth. Stir in chopped nuts, if using.
Immediately pour frosting over cake and use an offset spatula or butter knife to spread it out evenly over the surface. Allow to set for a few minutes before slicing and serving the cake.