Heat oven to 350℉/177℃.
Spray three 6-inch pans with baking spray. Set aside.
In a small bowl, carefully whisk cocoa and 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, buttermilk, oil, and vanilla.
Whisk in bloomed chocolate.
Pour wet ingredients over dry ingredients.
Whisk until combined.
Divide evenly among prepared pans.
Bake for 35 minutes, or until cake layers test done.