Light & Fluffy Chocolate Mousse Cake by Ash

Ingredients

• 2 large eggs
• 300ml double cream (heavy cream), divided
• 200g dark chocolate, chopped
• 400g milk chocolate, chopped, divided
• 4 egg whites
• 4 egg yolks
• 100g plain flour (all-purpose flour)
• 100g granulated sugar, divided
• 60g light brown sugar
• 25g cocoa powder
• 1/2 tsp instant coffee powder
• 40ml hot water
• 100ml vegetable oil (or other flavorless oil)
• 60ml buttermilk (or substitute)
• 1/2 tsp baking soda
• 1 tsp white vinegar (or apple cider vinegar, or lemon juice)
• 1/2 tsp cream of tartar
• 2 tsp vanilla extract
• 1/4 tsp salt

Instructions

1. Preheat oven to 180°C (350°F). Grease and line the bottom and sides of an 8-inch round cake tin with baking parchment. Set aside.
2. In a large mixing bowl, combine cocoa powder, instant coffee, and hot water. Whisk until completely smooth. This intensifies the chocolate flavor.
3. Add vegetable oil, buttermilk, and eggs to the cocoa mixture. Whisk until well combined and smooth.
4. Gradually add the plain flour, 60g granulated sugar, and light brown sugar to the wet ingredients. Whisk until you achieve a smooth batter, being careful not to overmix.
5. In a small separate bowl, combine baking soda and white vinegar. It will immediately bubble and fizz. Give it a quick stir and immediately whisk this into your cake batter until just incorporated.
6. Pour the cake batter into the prepared 8-inch cake tin. Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. If needed, level the top of the cooled cake later for a flat surface.
7. Once baked, remove the cake from the oven and let it cool completely in the tin on a wire rack. This is crucial before assembling the mousse cake.
8. For the mousse, in a large, chilled bowl, whip 300ml of double cream on high speed using an electric mixer until it forms smooth, thick, and firm peaks. Transfer the whipped cream to the refrigerator to keep it cold.
9. In a separate, very clean large bowl (ensure no grease), whip the egg whites and cream of tartar on medium speed. Once the egg whites become foamy, gradually add the remaining 40g granulated sugar, still whipping.
10. Continue whipping until you achieve stiff, glossy peaks. The egg whites should hold their shape firmly. Set aside.
11. In a third large, microwave-safe bowl, combine the dark chocolate and 200g milk chocolate (for the mousse). Microwave in 15-second intervals, stirring well after each, until completely melted and smooth. Be careful not to overheat.
12. Add the egg yolks, vanilla extract, and salt to the melted chocolate. Whisk vigorously until well combined. The mixture will become very thick in texture; this is normal.
13. To loosen the chocolate mixture, add a large dollop (about 1/4 cup) of the chilled whipped cream and whisk it thoroughly until the chocolate mixture is smooth and slightly thinned. This step prevents the chocolate from seizing when the rest of the cream is added.
14. Gently fold in the remaining whipped cream into the chocolate mixture using a spatula. Use a cutting and folding motion to maintain the airiness.
15. Add half of the whipped egg whites to the chocolate mixture and gently fold them in until just combined. Then, add the remaining whipped egg whites and fold until no visible lumps or streaks of egg white remain. Be very gentle to preserve the air.
16. To assemble, retrieve an 8-inch springform or loose-bottom cake tin. Line the bottom and sides with baking parchment (if not already done with the cake layer). Place the completely cooled chocolate cake layer into the bottom of the prepared tin.
17. Pour the chocolate mousse mixture evenly over the cake layer in the tin. Gently spread and smooth the top with an offset spatula. Place the cake into the freezer for at least 20 minutes to firm up slightly.
18. While the mousse chills, prepare the ganache. In a microwave-safe bowl, combine the remaining 200g milk chocolate (for the ganache) and 120ml double cream.
19. Heat in the microwave for 30 seconds, then remove and stir well. Continue heating in 15-second intervals, stirring after each, until the chocolate is fully melted and the ganache is smooth and glossy. If lumpy, you can optionally pass it through a fine-mesh sieve.
20. Remove the cake from the freezer. Carefully pour the warm, smooth ganache over the chilled mousse layer, spreading it gently to create an even top surface.
21. Transfer the assembled chocolate mousse cake to the refrigerator and chill overnight (at least 8 hours). This allows the mousse and ganache to fully set, ensuring clean slices.
22. The next day, carefully remove the cake from the springform tin. You may need to gently warm the sides of the tin with a warm towel or blow dryer for a few seconds if it's sticking. Decorate with additional whipped cream or fresh berries if desired, then slice and serve.