Preheat the oven to 350°F. Lightly spray a 9x13 inch baking pan with cooking spray and set aside.
In a medium bowl, mix together the sugar, flour, cocoa powder, baking powder, baking soda and salt. Add in the eggs, milk, vegetable oil and vanilla, and mix until fully incorporated.
Slowly mix in the boiling water. The batter will be very watery, and that is okay!
Pour the batter into the prepared baking pan, and bake in the preheated oven for 35-40 minutes, or until a toothpick inserted comes out clean. Allow to cool.
Meanwhile, in a medium bowl, cream together all of the frosting ingredients until smooth. Frost the cake once it is cool.