Make sure your cocoa and water mixture is still hot when adding to the batter! It results in a more moist cake.
Preheat the oven to 350°F. Line the bottom of each cake pan with parchment paper.
Sift the flour, baking powder, baking soda, and salt into a medium bowl and whisk together. Set aside.
Put the cocoa powder in a medium heatproof bowl. Bring 3 cups of water to a boil, then carefully measure exactly 2 cups and pour over the cocoa powder and whisk until completely smooth. Set aside to cool slightly.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar of medium speed until light and fluffy, about 8 minutes.
Add the eggs, two at a time, beating until each addition is incorporated before adding the next. Scrape down the sides of the bowl with a spatula when necessary.
Add the flour mixture in flour parts, alternating with the warm cocoa mixture in three parts, beating just until each addition is incorporated before adding the next; do not overmix.
Pour the batter into the prepared pan or pans. Bake until a toothpick inserted in the center comes out clean, about 45 minutes for the 9 x 13-inch pan or 55 minutes for the 8-inch pans, rotating the pan(s) halfway through.
Transfer to a wire rack and let cool completely in the pan(s). Cover tightly with plastic wrap and refrigerate overnight. Loosen the edges with a straight spatula, invert the pan(s) to remove the cake, and peel off the parchment.