Lightly spray a 9x13-inch cake pan with a non-stick spray
Combine flour, sugar, cocoa powder, baking soda, baking powder, and salt in a bowl.
2 cups flour, 2 cups sugar, ½ cup unsweetened cocoa powder, 2 teaspoons baking soda, 1 teaspoon baking powder, ½ teaspoon salt
Add buttermilk, oil, eggs, and vanilla. Mix until combined
2 eggs, 1 cup buttermilk, ½ cup oil, 1 teaspoon vanilla
Stir in boiling water and stir until well incorporated
1 cup boiling water
Add immediately into a greased 9x13-inch cake pan and put into a preheated 350°F oven.
Bake for 35-40 minutes until the cake is cooked through
Remove from oven and allow the cake to cool in pan
Icing
Melt butter in a saucepan over medium-low heat
½ cup butter
Turn off the heat and whisk in cocoa powder
⅓ cup unsweetened cocoa powder
Add buttermilk, vanilla and powdered sugar. Whisk until smooth. If small lumps of powdered sugar exist, just keep stirring until they disappear. The icing will be very thin and pourable.
3 cups powdered sugar, ½ cup buttermilk, 1 teaspoon vanilla
While the cake is still warm, pour icing over the cake. Level the icing with a spatula and allow icing to fully cool and set-up before slicing
Notes
The cake batter and icing can be completely mixed using a whisk
As soon as you add the boiling water, mix thoroughly, and place directly into the cake pan and into the oven. The boiling water helps develop flavors with the cocoa powder but also activates the baking soda and powder and gives the cake the proper rise.
Check to see if your cake is done by inserting a toothpick into the center of the cake. If it comes out clean, with no batter and just a few crumbs sticking to it, the cake is done. If there is batter, continue baking the cake for a few more minutes and test again.
If using chocolate icing, add it while the cake is still warm. The icing will become slightly firm once cool and will have a crackly surface. If cutting the cake too soon, the icing will run off each slice.
You may also use your favorite frosting. Allow cake to fully cool before frosting cake.