Ingredients
Equipment
Method
- Preheat your oven to 350°F (177°C). Grease four 8-inch or 9-inch cake pans, line them with parchment paper rounds, and grease the parchment paper. This helps the cakes release easily.
- In a large bowl, whisk together the flour, sugar, cocoa powder, espresso powder (if using), baking soda, baking powder, and salt. Set this dry mixture aside.
- In another large bowl, use a mixer to combine the sour cream, oil, and eggs on medium-high speed until blended. Add the buttermilk and vanilla extract and mix until combined. Pour the dry ingredients into the wet ingredients, add the hot water or coffee, and mix on low speed until just combined. Gently fold in the floured mini chocolate chips, if you are using them. Divide the batter evenly among the prepared pans.
- Bake for 19–23 minutes. Baking times can vary, so check for doneness by inserting a toothpick into the center; it should come out clean. Let the cakes cool in the pans on a cooling rack for 1 hour. Run a knife around the edges to loosen the sides, then remove the cakes from the pans. Peel off the parchment paper and place the cakes on the rack to finish cooling completely. The cakes may sink slightly in the middle as they cool, which is normal.
- To make the chocolate mousse: In a small bowl, whisk the hot water and cocoa powder until smooth. Melt the chopped chocolate using a double boiler or in the microwave in 20-second intervals, stirring between each, until smooth. Pour the hot water and cocoa mixture into the melted chocolate and stir until thick and smooth. Set aside. In a large bowl, whip the heavy cream, confectioners’ sugar, and vanilla extract on medium-high speed until medium peaks form, about 3 minutes. Gently fold the chocolate mixture into the whipped cream until just combined; do not overmix. Cover the mousse and refrigerate for at least 2 hours or up to 2 days.
- Assemble the cake: Level the cake layers if needed by slicing off the tops with a serrated knife. Place one cake layer on your serving plate. Spread about 1.5 cups of chocolate mousse evenly over the top. Add the second cake layer, upside down, and spread another 1.5 cups of mousse. Add the third cake layer, right side up, and spread with mousse. Top with the final cake layer. Spread the remaining mousse over the top and sides of the cake for a crumb coat. Use a bench scraper to smooth the sides. Refrigerate the cake for at least 1 hour and up to 4 hours.
- While the cake is chilling, prepare the chocolate ganache: Place the chopped chocolate in a heat-proof bowl. Heat the cream in a small saucepan over medium heat until it just begins to simmer. Pour the warm cream over the chocolate and let it sit for 2–3 minutes. Stir slowly with a metal spoon or spatula until the chocolate is melted and the mixture is smooth. Refrigerate the ganache for at least 30 minutes or up to 1 hour to thicken.
- Pour or spoon the thickened ganache over the chilled cake. Smooth the top and sides with an icing spatula. Add optional garnishes like fresh berries or chocolate shavings. Serve the cake immediately or chill, uncovered, for up to 4–6 hours before serving. The cake can be served at room temperature or chilled. Cover any leftover cake tightly and store in the refrigerator for up to 5 days.
Notes
This cake can be prepared in advance. Cake layers can be baked, cooled, wrapped tightly, and refrigerated for up to 2 days or frozen for up to 3 months. Thaw overnight in the refrigerator. The chocolate mousse and ganache can also be made ahead and refrigerated. Bring refrigerated components to room temperature before assembling. Leftover cake can be stored, tightly covered, in the refrigerator for up to 5 days. For a deeper chocolate flavor, consider using espresso powder or coffee in the cake batter, as it enhances the chocolate taste without making the cake taste like coffee. Use natural cocoa powder for the cake; a blend with Dutch-process is acceptable, but avoid using only Dutch-process. For the mousse, either natural or Dutch-process cocoa powder works. Ensure all refrigerated ingredients like sour cream, eggs, buttermilk, and heavy cream are at room temperature for even batter mixing. If you don't have buttermilk, you can make a substitute by adding 1 teaspoon of white vinegar or lemon juice to a measuring cup and filling it to 1/2 cup with whole milk; let it sit for 5 minutes before using. Use quality semi-sweet chocolate baking bars for the best results in the mousse and ganache; chocolate chips can be used if necessary.