The Best (Texas) Chocolate Sheet Cake

CAKE:

• 2 cups all-purpose flour
• 2 cups sugar
• ¼ teaspoon salt
• 4 heaping Tablespoons unsweetened baking cocoa
• 2 sticks butter
• 1 cup boiling water
• ½ cup buttermilk (Or use this tip to make a buttermilk substitute: Place 1/2 Tablespoon white vinegar or lemon juice in a liquid measuring cup. Add enough regular milk to bring the liquid up to the 1/2 cup line. Let stand for 5 minutes, then use in the recipe.)
• 2 large eggs, beaten
• 1 teaspoon baking soda
• 1 teaspoon vanilla

FROSTING:

• 1 3/4 sticks butter
• 4 heaping Tablespoons unsweetened baking cocoa
• 6 Tablespoons milk
• 1 teaspoon vanilla
• 1 pound (minus 1/2 cup) powdered sugar (about 3 to 3 1/4 cups)
• 1/2 cup finely chopped pecans or walnuts, optional (Jane did not use nuts in her frosting and it was great…so if you don’t like nuts, leave them out!)

In a large mixing bowl, combine flour, sugar, and salt.

Melt butter in a saucepan and add cocoa. Stir together. Add boiling water, allow mixture to boil for 30 seconds, then turn off heat. Pour over flour mixture, and stir lightly to cool.

In a measuring cup, pour the buttermilk and add beaten eggs, baking soda, and vanilla. Stir buttermilk mixture into butter/chocolate mixture. Pour into 13 x 18″ sheet cake pan and bake at 350 degrees for 20 minutes (if using a 10 x 15″ jelly roll pan, you may need to bake 22 to 23 minutes). NOTE: I greased my pan, but the Pioneer Woman’s recipe doesn’t say to do that, so I guess that’s optional.

While cake is baking, make the icing. If using nuts, chop them finely. Melt butter in a saucepan. Add cocoa, stir to combine, then turn off heat. Add the milk, vanilla, and powdered sugar. Stir together. Add the nuts, stir together, and pour over warm cake. (NOTE: if you follow these directions, you’ll notice your frosting is still a little lumpy. To make her frosting really nice and smooth, Jane simply cooked the frosting several minutes longer over low heat, stirring until the mixture was smooth; she also omitted the nuts. You could also leave the nuts out of the frosting mixture and sprinkle them on top of the icing instead). If you made your cake in a 13 x 18″ pan, pour on all the frosting–you may need to spread it a little so it covers the entire cake; if your cake is in a 10 x 15″ pan, just pour enough frosting on to cover the surface of the cake without going over the edges of the pan. You’ll probably have about 1/2 to 2/3 cup of frosting left over.

You can let the frosting cool and set before cutting into squares if you want, or cut into squares while it’s still warm and dig in! The frosting will start setting in about a half hour, but it needs between 1 and 2 hours to set more firmly; at 2 hours, it should be nice and firm.

(Source)