Chocolate Cake:

1. Preheat the oven to 165°C (325°F) and prepare three 6-inch (15 cm) cake pans by lining only the bottom with parchment paper.
2. In a small bowl, mix the buttermilk with the warm coffee and set aside.
3. In a large mixing bowl, sift together the dry ingredients: flour, cocoa powder, baking soda, baking powder, and salt.
4. In another large mixing bowl (or the bowl of a stand mixer), whisk together the sugar, vegetable oil, eggs, and vanilla extract using a hand whisk or an electric mixer.
5. Gradually add the buttermilk/coffee mixture and the dry ingredients in alternating additions, mixing after each until you get a smooth and slightly liquid batter.
6. Divide the batter evenly into the three prepared cake pans, or bake in batches if using a single pan.
7. Bake for about 40 minutes, or until a toothpick inserted into the center comes out clean.
8. Let the cakes cool for a few minutes, then run a knife around the edges and carefully unmold them. Place them upside down on a wire rack to cool completely.
9. Use a cake leveler or a serrated knife to trim the domed tops and create three even cake layers.

Chocolate Buttercream

1. Follow the chocolate buttercream frosting recipe and keep it at room temperature until ready for assembly.

Assembly

1. Place the first chocolate cake layer on a cake board and set it on a cake turntable.
2. Spread a generous layer of chocolate buttercream using an offset spatula, then place the second cake layer on top and repeat. Finish with the third layer.
3. Apply a thin crumb coat of buttercream over the entire cake to trap any crumbs, then chill for 20 minutes.
4. Mix the buttercream with a spatula to smooth it out, then apply a final layer of frosting and use a cake scraper for a clean finish.
5. Decorate with sprinkles or chocolate shavings, then serve immediately or refrigerate. If chilled, let the cake sit at room temperature for about an hour before serving for a softer buttercream texture.