1️⃣ Preheat the oven: Preheat oven to 350°F (177°C). Grease four 8-inch or 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper. 🧽

2️⃣ Prepare the cake batter: Whisk the flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder (if using) together in a large bowl. In another bowl, mix the oil, eggs, and sour cream together on medium-high speed until combined. Add the buttermilk and vanilla and mix until combined. Pour the wet ingredients into the dry ingredients, add the hot water/coffee, and whisk or beat on low speed until the batter is completely combined. Fold in the chocolate chips, if using. 🍫

3️⃣ Bake the cakes: Divide batter evenly between 4 pans. Bake for 19-23 minutes. The cakes are done when a toothpick inserted in the center comes out clean. Remove the cakes from the oven and set on a wire rack. Allow to cool completely in the pan. 🕰️

4️⃣ Prepare the chocolate mousse: Whisk the hot water and cocoa powder together. Melt the chopped chocolate bars in a double boiler or microwave. Pour the hot water/cocoa mixture into melted chocolate and stir until thick and smooth. Using a hand mixer or stand mixer, whip the heavy cream, confectioners’ sugar, and vanilla extract together until medium peaks form. Fold in the chocolate mixture gently. Chill in the refrigerator for at least 2 hours and up to 2 days. 🍫❄️

5️⃣ Assemble the cake layers: Level the cakes if needed. Place 1 cake layer on your cake stand and evenly cover the top with about 1.5 cups chocolate mousse. Repeat with the 2nd and 3rd layers. Top with the final 4th cake layer. Spread any remaining mousse around the sides as a crumb coat. Refrigerate cake for at least 1 hour and up to 4 hours. 🍰

6️⃣ Prepare the ganache: Place chopped chocolate in a medium heat-proof bowl. Heat the cream in a small saucepan over medium heat until it begins to gently simmer. Pour over chocolate, then let it sit for 2-3 minutes. Stir until chocolate has melted and mixture is smooth. Let it cool for 20 minutes at room temperature before spreading on the chilled cake. 🍫

7️⃣ Finish the cake: Pour/spoon ganache on chilled cake. Smooth the top and sides with an icing spatula. Top with optional garnish such as fresh berries and/or chocolate shavings. Serve cake immediately or chill for up to 4-6 hours before serving. 🍓🍫

8️⃣ Storage: Cover leftover cake tightly and store in the refrigerator for up to 5 days. 🍰❄️