Ingredients
Equipment
Method
- Preheat your oven to 350°F (177°C). Grease four 8-inch or 9-inch cake pans. Line the pans 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 whisk or beat 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 might sink slightly in the middle as they cool, which is normal.
- To make the chocolate mousse: In a small bowl, whisk together the hot water and cocoa powder until smooth. Melt the chopped chocolate in a double boiler or in the microwave. If using the microwave, heat in 20-second increments, stirring after each, until melted and smooth. Pour the hot water/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 with a mixer 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 before making the ganache. If needed, level the cakes by slicing off the tops with a serrated knife to create a flat surface. Place one cake layer on your serving plate. Spread about 1.5 cups of chocolate mousse evenly over the top. Place the second cake layer upside down on top of the mousse 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.
- To make 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. Use an icing spatula to smooth the top and sides. 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
You can prepare the cake layers ahead of time. 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 in advance and refrigerated. Bring them to room temperature before assembling the cake. To freeze the assembled 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. Natural cocoa powder is recommended for the cake batter; avoid using only Dutch-process cocoa powder. For the mousse, either natural or Dutch-process cocoa powder works. Using room temperature ingredients ensures the batter mixes evenly. If you don't have buttermilk, you can make a substitute by adding 1 teaspoon of white vinegar or lemon juice to a liquid measuring cup and filling it to the 1/2 cup line with whole milk. Let it sit for 5 minutes before using.