Heat the oven to 350°F (175°C). Grease two 9-inch round cake pans and line the bottoms with parchment paper.

Whisk the flour, sugar, cocoa powder, baking powder, baking soda, and salt together in a large bowl until evenly combined.

Add the eggs, buttermilk, vegetable oil, and vanilla. Beat for approximately 1–2 minutes, until the mixture is smooth.

Gradually mix in the boiling water or hot brewed coffee. The batter will be quite thin, which is normal and helps create a moist cake.

Divide the batter evenly between the prepared pans. Bake for approximately 30–35 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.

Cool the cakes in their pans for about 10 minutes, then turn them onto wire racks. Let them cool completely for 60 minutes or longer before adding ganache.

Place the finely chopped chocolate in a heatproof bowl. Heat the cream until steaming and just beginning to simmer, then pour it over the chocolate and let it stand for 5 minutes.

Whisk the ganache from the center outward until smooth. Stir in the butter and salt if using, then let it cool at room temperature until thick enough to spread.

Level the cake layers if necessary. Place one layer on a serving plate, spread it with approximately 1 cup of ganache, add the second layer, and cover the top and sides with the remaining ganache.

Decorate with berries, chocolate shavings, sprinkles, powdered sugar, or small chocolate hearts. Chill briefly if you prefer a firmer ganache finish.