Best Lunch Lady Chocolate Sheet Cake with Fudge Icing

Note: The fudge icing must be poured on a warm cake. Read the instructions through for timing.
Grease a 10 X 15 baking pan.
Preheat oven to 350°F.
Add the flour, sugar, baking soda, baking powder, expresso powder, and salt into a bowl and whisk to combine.
Next, add the butter, water, and cocoa powder into a saucepan and bring to a boil whisking constantly until smooth. Immediately remove from the heat and let cool.
Whisk together the buttermilk, eggs, and vanilla extract in a small bowl and set aside.
Pour the cooled cocoa powder mixture into dry ingredient mixture and stir to combine. Pour in the buttermilk mixture and stir for one minute.
Carefully pour the Lunch Lady Chocolate Sheet Cake batter into the prepared pan. Spread evenly and gently.
Bake for 20 to 25 minutes or until a toothpick comes out clean.
Place on a rack.

While the cake is baking sift the cocoa powder, powdered sugar, and salt together in a large mixing bowl to remove lumps. Set aside. (See notes.)
Add the butter and milk into a saucepan.
When the chocolate sheet cakes comes out of the oven, bring the butter and milk to simmering stirring often. Remove from the heat and stir in the vanilla.
Pour the hot mixture into over the cocoa powder mixture and stir with a wooden spoon or rubber a spatula until smooth. (See notes.) Immediately pour the icing over the warm Best Lunch Lady Chocolate Cake.
Let the cake cool completely through.
Serve in square slices from the pan for charming nostalgia.
Cover with foil and store in the refrigerator for up to 7 days. It is delicious cold with a glass of cold milk.