Preheat the oven to 350°F / 175°C. Grease then line four 7-inch or three 8-inch cake pans with nonstick spray and parchment paper rounds. Set aside. Add 1 cup of unsalted butter into the bowl of a stand mixer or a large bowl and beat on a medium speed until smooth and creamy. Add 2 1/2 cups sugar and cream together on a high speed for 1-2 minutes with a whisk attachment or hand mixer. Scrape the sides of the bowl as needed. The mixture should become light in color.
Then mix in 4 large eggs on a medium speed, two at a time.
In a separate bowl combine 1 cup of hot water, 1 cup of buttermilk, and 1 cup of black cocoa. Whisk until the mixture is smooth and no clumps remain.
Sift 3 cups of flour, 1 1/2 tsp baking powder, 1 tsp baking soda, and 1 tsp salt into a large bowl.
Next, add in 1/3 of the dry ingredients into the butter/egg mixture. Scrape the sides of the bowl with a spatula to ensure the flour gets fully incorporated.
Pour in half of the buttermilk/cocoa mixture and gently pulse the mixture to incorporate the liquid. If you don't, you'll end up splattering the cocoa mixture all over your kitchen!
Mix in the 2nd portion of the dry ingredients on a low speed. Pour in the remaining buttermilk/cocoa mixture and again gently pulse. Then mix on low until fully combined.
Add in the last portion of the dry ingredients and mix on low until fully incorporated.
Divide the batter evenly between the prepared pans. I like to use a kitchen scale to make sure each pan has the same amount of batter.
Bake for 32-35 minutes or until a toothpick comes out with a few moist crumbs.
Allow layers to cool in the pans for about 10 minutes, then run an offset spatula around the edge of the pans to help separate the layers from the pans.
Place the pans in the freezer for about 45 minutes, to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers from the pans.
Use a serrated knife to level the tops of the layers, then set aside. If you're making these in advance, you can wrap and freeze them at this point.