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.
- 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 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. 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. Loosen the sides with a knife, remove the cakes from the pans, peel off the parchment paper, and let them finish cooling completely on the rack. The cakes may sink slightly in the middle as they cool; this is normal.
- 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. If using the microwave, heat in 20-second increments, stirring after each, until melted and smooth. Stir the hot water/cocoa mixture into the melted chocolate 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. 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: Level the cakes 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. Place the second cake layer upside down on top and spread with another 1.5 cups of mousse. Add the third cake layer right side up and cover with the remaining mousse. Spread the remaining mousse on 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.
- Make the chocolate ganache: Place the chopped chocolate in a heat-proof bowl. Heat the cream in a 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 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. Leftover cake can be covered tightly and stored in the refrigerator for up to 5 days.
Notes
For the cocoa powder, natural cocoa powder will give you a more intense chocolate flavor, while Dutch-process cocoa powder will result in a darker color and milder taste. If your cakes sink in the middle, this is expected and provides a good base for the mousse. Ensure your heavy cream is very cold for the best whipping results.