Heat oven to 350℉/177℃.
Spray four 8-inch round cake pans with cooking spray. Set aside.
In a small bowl, combine cocoa and boiling water. Set aside.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt. Set aside.
In a medium bowl, whisk together eggs, vegetable oil, milk, and vanilla.
Pour egg mixture into flour mixture and whisk until well blended, about a minute.
Stir in chocolate.
Divide batter evenly among pans.
Bake for 20-25 minutes, until cake tests done.