Grease 2 x 22cm cake pans with butter, then line the base. (see spring form pans note below).
Batter:
Sift flour, cocoa, baking powder and baking soda into a large bowl. Add Sugar and salt. Whisk briefly to combine.
Add eggs, milk, oil and vanilla. Whisk well to combine until lump free - about 30 seconds.
Add boiling water and whisk to incorporate. The batter is VERY thin (see video).
Pour batter into cake pans.
Baking:
Bake for 35 minutes or until a wooden skewer inserted into the centre comes out clean. See Note 4 regarding cook time if pans are on different shelves.
Cool for 10 minutes, then turn out onto wire racks upside down (Note 5).