Method For Chocolate Sponge:

1.  Preheat the oven to 180C (OTG mode: upper rod + lower rod + fan). Prepare a 5.5-inch round ring by cling-wrapping it twice, then grease it with some oil and place butter paper at the bottom.
2. Now, in a small bowl, mix milk and apple cider vinegar to make buttermilk and keep it aside for later use.
3. In a bowl, sieve all the dry ingredients, which are flour, baking powder, baking soda, sugar, natural cocoa powder, salt and salt.
4. In a different bowl, mix all the liquid ingredients, which are eggs, hot water, oil, buttermilk and vanilla essence.
5. Now add the liquid mixture to the dry mixture gradually and mix to get a smooth and lump-free batter.
6. Transfer the batter to the prepared ring and bake at 180C (OTG Mode: Lower rod only) for 20 minutes or until the toothpick inserted in the middle comes out clean.

Method For Dark Chocolate Mousse:

1. Soak the gelatin sheet in cold water and keep it aside.
2. Now, in a bowl, melt chocolate and warm fresh cream simultaneously.
3. Add fresh cream to the melted chocolate to make the ganache. 
4. Squeeze the soaked gelatin sheet to remove the excess water, add it to the ganache, and mix.
5. Microwave the ganache again for 5 seconds, then keep mixing until the gelatin sheet is dissolved completely.
6. Now beat whipping cream until it reaches the soft peak stage, then fold in the ganache to it.
7. Pour the mousse over the baked and cooled sponge, then keep it in the freezer for 2 hours.

Method For White Chocolate Mousse:

1. Soak the gelatin sheet in cold water and keep it aside.
2. Now, in a bowl, melt white chocolate and warm fresh cream simultaneously.
3. Add fresh cream to the melted chocolate to make the ganache. 
4. Squeeze the soaked gelatin sheet to remove the excess water, add it to the ganache, and mix.
5. Microwave the ganache again for 5 seconds, then keep mixing until the gelatin sheet is dissolved completely.
6. Now beat whipping cream until it reaches the soft peak stage, then fold in the ganache to it.
7. Pour the white chocolate mousse over the baked and cooled sponge, then keep it in the freezer for 2 hours.

Method For Frosting:

1. In a bowl, melt the dark chocolate and warm fresh cream simultaneously.
2. Add fresh cream to the melted chocolate to make the ganache. 

Method For Assembly:

1. Once all the layers are set properly, frost the entire cake with ganache.
2. Now, drizzle melted white chocolate on top.
3. Stick chocolate curls on the sides of the cake.
4. Pipe ganache at the top edge of the cake using open star nozzle.
5. Garnish the top with golden metallic sprinkles.