Begin by preheating your oven to 350°F (175°C). This ensures that your cake will bake evenly.
Grease and flour three 9-inch round cake pans. This will help the cake layers release easily after baking.
In a small bowl, combine 1 cup of lukewarm water with the unsweetened cocoa powder. Stir until smooth, then set aside to cool slightly.
In a large mixing bowl, whisk together the cake flour, baking soda, granulated sugar, and brown sugar until well combined.
In a separate bowl, beat the room-temperature butter with an electric mixer until creamy and light in color. This should take about 2-3 minutes.
Add the eggs one at a time, mixing well after each addition. This will help ensure your batter is smooth and well-mixed.
To the butter and egg mixture, add the buttermilk and the cooled cocoa mixture. Mix until fully combined.
Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Be careful not to overmix; a few lumps are okay.
Divide the batter evenly among the three prepared pans and smooth the tops. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, remove the cake layers from the oven and let them cool in the pans for 10 minutes. Then, transfer them to a cooling rack to cool completely.
Once the layers are completely cool, frost between the layers and on top using your favorite chocolate frosting. Enjoy a slice of your homemade Triple Layer Cocoa Cake!