Mix the flour, sugar, and baking soda in a large bowl. Set aside.
In a separate small bowl, whisk the 2 eggs and set aside.
Add the butter, shortening, cocoa powder, water, vanilla, and buttermilk to a saucepan, and place it on your stovetop over medium heat. Stir until the ingredients are all melted together, and the mixture has just started to boil.
Combine the melted mixture with the dry ingredients. Stir, then add the 2 eggs and mix until combined.
Pour the batter into a greased and floured sheet pan. Bake at 400º for 12-20 minutes. (The cooking time will vary depending on the depth of your cake pan, so it’s better to just keep an eye on it.)