Preheat your oven to 180°C (350°F). Grease and line an 8-inch cake tin and set it aside. In a large bowl, whisk together the cocoa powder, instant coffee, and hot water until well combined. Add in the vegetable oil, buttermilk, and eggs, and whisk again until everything is blended. Sift in the all-purpose flour, granulated sugar, and light brown sugar, then whisk until you have a smooth batter. In a small separate bowl, combine the baking soda and vinegar – this will fizz. Quickly mix and add this mixture to the batter, whisking it in thoroughly. Pour the batter into your prepared tin and bake for around 30 minutes, or until a toothpick comes out clean from the center. Once baked, let the cake cool completely before proceeding.

While the cake cools, start making the white chocolate mousse. In a large bowl, whip the egg whites until they become foamy. Gradually add in the granulated sugar, whipping continuously until stiff peaks form; set aside. In a separate bowl, whip the heavy cream until you get firm peaks and leave this aside too. In a third large bowl, whisk together the melted white chocolate, egg yolks, salt, and vanilla extract—don’t worry if it seems very thick, that's normal. Add a spoonful of the whipped cream to the chocolate mixture to loosen it, then gently fold in the remaining whipped cream. Next, fold in half of the whipped egg whites, followed by the other half, mixing gently until fully combined without lumps or streaks.

Line the bottom and sides of a tall loose-bottom or springform cake tin with baking paper. Level the cooled cake layer by slicing off the domed top, then place the cake into the prepared tin to form an even layer. Spread the white chocolate mousse (from Step 2) evenly over the cake. Place the tin in the freezer for at least 20 minutes to allow the mousse to set before adding the next layer. I always recommend letting each mousse layer chill fully to keep the layers distinct and beautiful in the final dessert.

While the first mousse layer chills, prepare the strawberry mousse. Mix the freeze-dried strawberry powder with the hot heavy cream until fully combined. In a large bowl, whip the second set of egg whites until foamy, then gradually add the granulated sugar and continue whipping until stiff peaks form; set aside. In another bowl, whip the heavy cream until firm peaks form. In a third bowl, whisk together the melted white chocolate, egg yolks, salt, and vanilla extract. Stir the strawberry mixture into the chocolate-egg yolk mixture. Add a spoonful of whipped cream to loosen, then fold in the rest of the whipped cream gently. Fold in half the whipped egg whites, then the remaining half, to create a smooth, airy strawberry mousse.

Once the white chocolate mousse layer has chilled for at least 20 minutes, gently spread the prepared strawberry mousse (from Step 4) on top. Return the cake to the freezer for another 20 minutes to set the strawberry layer before adding the ganache.

To make the dark chocolate ganache, place the dark chocolate and heavy cream in a bowl. Microwave for 30 seconds, then stir. Continue heating in 15-second bursts, stirring after each, until the chocolate is melted and the mixture is smooth and glossy. Pour the ganache over the chilled strawberry mousse layer, making sure it spreads evenly. Place the assembled cake in the fridge and chill overnight for the best texture and clean slices. I like allowing the flavors to meld overnight for a more harmonious taste.

The next day, remove the cake from the tin and prepare for serving. You may optionally pipe whipped cream on top and sprinkle some extra freeze-dried strawberry powder as decoration. Slice and enjoy this beautiful layered mousse cake!