Preheat oven to 350 F. Grease and flour two 9-inch round cake pans; set aside.
In a small bowl, whisk together cocoa powder and hot chocolate; set aside.
In a large bowl, whisk eggs, butter, buttermilk, and vanilla together until well-combined. It's important that the eggs and buttermilk are at room temperature so that the melted butter doesn't clump up.
Add hot chocolate mixture, and whisk to combine.
Whisk in flour, sugar, baking soda, and salt just until incorporated.
Divide batter evenly between prepared pans.
Bake for 20-35 minutes, or until wooden toothpick inserted in center comes out clean.
Cool in pans for 10 minutes.
Remove cakes from pans and cool on wire rack until room temperature.
The cake is now ready to frost and assemble.
I use regular unsweetened baking cocoa powder, not Dutch-processed.
I use 1 cup of hot water mixed with 1.25 oz. of hot chocolate powder (standard-sized packet).