Preheat over to 400°.
Sift or whisk together in a large mixing bowl 2 cups of flour and 2 cups of sugar. Set aside.
In a saucepan, melt over medium heat 1 stick butter, 1/2 cup shortening, 3 T. cocoa powder, and 1 cup water. Bring to a boil and remove from heat.
Pour melted mixture over the flour and sugar.
Whisk together in a measuring cup the buttermilk, eggs, baking soda, vanilla, and salt.
Pour the buttermilk mixture into the flour and sugar mixture. Whisk to combine.
Pour into a greased 9 x 13 inch baking dish.
Bake in oven for 30 minutes. Start testing after 25 minutes. Do not overbake. You still want a small amount of batter to cling to toothpick or cake tester when inserted into cake.
When cake is cool, frost with Sylvia's Chocolate Icing.