Preheat your oven to 350°F (175°C).
Grease and flour a 13x18 inch baking sheet.
In a large bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, and salt.
Add eggs, milk, oil, and vanilla to the dry ingredients.
Beat the mixture on medium speed for 2 minutes.
Stir in the boiling water (the batter will be thin).
Pour the batter into the prepared baking sheet.
Bake for 30 minutes or until a toothpick comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack.
Once cooled, frost with your favorite chocolate frosting.