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. Parchment paper helps the cakes release easily.
- Make the cake: In a large bowl, whisk together the flour, sugar, cocoa powder, espresso powder (if using), baking soda, baking powder, and salt. Set this bowl aside.
- In another large bowl, use a handheld or stand mixer to combine the sour cream, oil, and eggs on medium-high speed until mixed. Add the buttermilk and vanilla and mix until combined. Pour the dry ingredients into the wet ingredients, add the hot water or coffee, and whisk or beat on low speed until combined. 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 your cakes. They are done when a toothpick inserted into the center comes 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, remove the cakes from the pans, peel off the parchment, and place them on the rack to finish cooling. The cakes might sink slightly in the middle as they cool, which is normal.
- Make the chocolate mousse: In a small bowl, whisk the hot water and cocoa powder until smooth. Melt the chopped chocolate in a double boiler or using the microwave. If using the microwave, place the chopped chocolate in a heat-proof bowl and microwave in 20-second intervals, stirring after each, until completely melted and smooth. Pour the hot water and cocoa mixture into the melted chocolate and stir until thick and smooth. Set aside. In a large bowl, use a handheld or stand mixer to whip the heavy cream, confectioners’ sugar, and vanilla extract on medium-high speed until medium peaks form, about 3 minutes. Medium peaks are between soft peaks and stiff peaks. Pour in the chocolate mixture and gently fold it together with a spatula. Do not overmix, as this can deflate the mousse. Cover the mousse and refrigerate for at least 2 hours or up to 2 days. This makes about 4.5 cups.
- Assemble the cake before preparing the chocolate ganache: Level the cakes if needed by slicing a thin layer off the tops with a serrated knife to create a flat surface. Place one cake layer on your cake stand or serving plate. Spread about 1.5 cups of chocolate mousse evenly over the top. Place the second layer upside down on top and spread with the same amount of mousse. Add the third cake layer, right side up, and spread with mousse. Top with the final cake layer. Spread the remaining mousse on the top and sides of the cake as a crumb coat. Use a bench scraper to smooth the sides. Refrigerate for at least 1 hour and up to 4 hours. Prepare the ganache while the cake is chilling.
- Make the chocolate ganache: Place the chocolate in a medium heat-proof bowl. In a small saucepan, heat the cream over medium heat just until it begins to simmer gently. Do not let it boil rapidly. Pour the warm cream over the chocolate and let it sit for 2–3 minutes to soften the chocolate. Stir slowly with a metal spoon or spatula until the chocolate has melted and the mixture is smooth. If the chocolate is not melting, do not microwave it. 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 leftover cake tightly and store in the refrigerator for up to 5 days.
Notes
You can prepare the cake layers in advance. Wrap cooled cake layers tightly and refrigerate for up to 2 days or freeze for up to 3 months. Thaw overnight in the refrigerator. The chocolate mousse and ganache can also be made ahead and refrigerated. Bring them to room temperature before assembling the cake. To freeze the finished cake, wrap it tightly and freeze for up to 3 months. Thaw overnight in the refrigerator. Espresso powder or coffee enhances the chocolate flavor without making the cake taste like coffee. For the cake, use a blend of natural cocoa powder and Dutch-process or Hershey’s “Special Dark” cocoa powder for the best flavor. Ensure you use acidic natural cocoa powder. For the mousse, either natural or Dutch-process cocoa powder works. Using room temperature ingredients helps the batter mix evenly. You can substitute plain yogurt for sour cream, though the cake may be less rich. If you don't have buttermilk, make a substitute by adding 1 teaspoon of white vinegar or lemon juice to a measuring cup, then adding whole milk to reach 1/2 cup. Let it sit for 5 minutes before using. For the mousse and ganache, use quality semi-sweet chocolate baking bars for best results, though high-quality chocolate chips can be used.