Preheat oven to 350°F. Lightly grease a 13"x18" baking sheet and set aside.
In a large mixing bowl combine flour and sugar.
2 cups flour, 2 cups sugar
In a pot add water, cocoa powder and butter. Heat on medium-low heat till butter is melted, then allow to boil for 30 seconds.
1 cup water, 4 tablespoons cocoa powder, 1 cup butter
Remove mixture from heat and add to bowl of dry ingredients.
In small bowl stir together baking soda and milk. Add milk mixture to the larger mixing bowl.
1 teaspoon baking soda, ½ cup milk
In empty, smaller bowl beat together whole eggs and yolks. Add egg mixture to the larger mixing bowl along with vanilla and stir gently to combine.
2 whole eggs, 2 egg yolks, 1 teaspoon vanilla
Transfer batter to greased baking sheet and place in oven to bake for 25 minutes.
While cake is baking prepare the frosting in the last 5 minutes. Add butter and evaporated milk to a pot on the stove. Allow to come to a boil, stirring occasionally. Add cocoa powder and sugar to the milk mixture, stir together and heat until boiling.
1 cup butter, ⅔ cup evaporated milk, 4 tablespoons cocoa powder, 1 cup sugar
Pour chocolate sauce into a mixing bowl. Add vanilla and powdered sugar and mix together.
1 teaspoon vanilla, 3 cups powdered sugar
If wanting an extra moist cake poke small holes over the top with a fork. Pour warm frosting over the hot cake and spread icing evenly over the top before it cools.
Carefully pick up sheet pan and drop lightly on counter to help frosting fall into the holes. Cut into slices and enjoy.