Position the oven rack that you will be placing your jelly roll pan on to the second to highest level in the oven; that is, it should be one rack-level away from the top of the oven. Preheat the oven to 350 degrees (F).
In a large mixing bowl, combine flour, sugar, and salt. Set aside.
Melt 2 sticks regular (NOT UNSALTED) butter in a medium saucepan.
While the butter is melting, boil 1 cup of water in a small pot. Set aside.
When the butter has melted, add 4 heaping, unleveled tablespoons of cocoa powder to the butter mixture. Mix thoroughly.
Keep the heat on (but make sure the butter mixture does not begin to bubble), pour in the boiling water.
Allow the resulting mixture to bubble for 30 seconds, before turning off the heat and setting aside.
Pour this chocolate-butter mixture into your large mixing bowl containing your flour mixture (from step 1), and stir lightly just enough to cool the chocolate.
In a measuring cup, pour 1/2 cup buttermilk.
Add 2 beaten eggs to the buttermilk in the measuring cup.
Add 1 teaspoon vanilla to the buttermilk-egg mixture in the measuring cup.
Add 1/2 teaspoon of baking soda to the buttermilk-egg mixture, and stir.
Pour the buttermilk-egg mixture into your large mixing bowl containing the flour and chocolate mixture. Stir together well until a smooth, consistent batter is achieved.
Pour the batter into a slightly greased 17.25 x 11.5 inch jelly roll pan, using a rubber spatula to spread evenly throughout the pan.
Place the jelly roll pan into the oven rack that you positioned in the first step, allowing it to bake at 350 degrees (F) for 20 minutes.
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Prepare the chocolate frosting while the cake is baking in the oven. Once the cake has finished baking in the oven, let cool slightly on a cooling rack before pouring the chocolate frosting on the still-warm cake.