Grease and line a 9 inch / 22cm round springform tin with baking paper. Preheat the oven to 180C / 350F / 160C fan forced.
Combine the butter and chocolate in a saucepan over low heat and stir until everything is melted and you have a smooth liquid. Allow to cool for a few minutes.
In a medium bowl, sift together the flour, cocoa, baking powder and salt. Add both sugars and whisk together until there are no lumps.
Combine the hot water and coffee and mix to dissolve.
In a large bowl, whisk together the cooled chocolate and butter mixture, eggs, coffee, milk and vanilla until well combined.
Add the wet ingredients to the dry ingredients and stir until just combined.
Pour the batter into the prepared tin and bake for around 25-28 minutes, until a toothpick inserted comes out with just a crumb or two attached.
Using a clean tea towel, very gently press down all over the top of the cake to flatten out any dome and press out a little to the edges to make sure the sides of the cake are right up against the tin. This step is optional but it will give your cake nice straight layers. Leave it in the tin to cool.
FOR THE CHOCOLATE MOUSSE (only once the cake has cooled)
Pour 1/2 cup cream into a small saucepan and the remaining cream into a glass bowl and place in the fridge.
Heat the 1/2 cup of cream, chocolate, butter and sugar over very low heat and stir until almost everything is melted. Remove from heat as soon as just a few lumps are left and stir until you have a smooth ganache. Pour it straight into a glass bowl and let it cool to room temperature.
Once the ganache has cooled, beat the remaining 2 cups cream to soft peaks. Using a spatula, stir about 1/2 cup of the whipped cream straight through the ganache to lighten it. Now fold the remaining cream, in two parts, into the mousse until just mixed through. Be very careful not to overmix the mousse or you'll knock the air out, making it dense.
Spread the mousse over the cake base, then smooth out the top with an offset spatula. Chill 2-3 hours.
FOR THE CHOCOLATE MIRROR GLAZE (this can take up to an hour to cool to the right consistency).
Sprinkle the gelatine over ¼ cup of water and allow it to ‘bloom’ for 5 minutes while you get on with the rest.
Combine the other ¼ cup of water, the cream, sugar and cocoa in a saucepan over low heat. Whisk to combine then use a spatula to stir over low heat, until the sugar has dissolved completely. Keep the heat low, it must not come to a boil.
Add the gelatine and water mixture and whisk in thoroughly.
Pour the glaze through a sieve into a jug (to remove any lumps). Now place the jug in the fridge or even the freezer until it cools enough to coat a metal spoon without running straight off. If there is a light film or skin on the top of the glaze, make sure to skim that off with a spoon before using.
Remove the outer frame of the spring form tin and the baking paper carefully. Optional extra step: If you want to smooth the mousse out a little around the sides and top see the notes section.
With the cake still on the baking tin base, sit it on top of a flat bowl inside a larger flat bowl or plate with high sides. Pour the glaze over the cake and use a large flat metal spatula to smooth it and push it over the edges, being careful not to press into the mousse. Remove the cake to drain the bowl back into the jug and repeat a second or even third time.
Leave the cake in the fridge to set for at least an hour before carefully removing the base. The tin base should come away fairly easily, then holding the cake underneath with one hand, use the other to peel back the baking paper halfway. Swap hands and remove the paper entirely. Use a sharp knife to run around the bottom edge to tidy up where the set glaze is not straight. Transfer to a cake plate and serve.
Cocoa & hot water: I use a standard Australian 20ml tablespoon (1 Tbsp = 4 tsp)
This cake should be served straight from the fridge.
I recommend making this cake a day ahead due to a number of cooling and setting times.
Baking time: This will depend on your oven, check it as you get close to the suggested time.
Feel free to use darker chocolate for the mousse, but be aware it will set to a firm consistency.
If you want to smooth the mousse out a little around the sides and top, heat an offset spatula either by running under hot water, then drying it off or use a kitchen blowtorch. Very gently run the heated spatula over the mousse to smooth it.
The glaze can take up to an hour to cool down enough to pour over the cake. Once it coats a spoon thickly, without all running off, the glaze is ready. It does not need to be very cold.
Once cold, the glaze will turn into a chocolate jelly. This can simply be reheated in the microwave to get it back to pouring consistency again if needed.
If you'd like to add the gold drizzle like this one, just mix a little edible gold glitter with melted white chocolate and drizzle over the top.
This cake will keep for 4-5 days in the fridge, in an airtight container.
Changes to the video: Due to requests for a moister cake, this recipe has been updated. Slight changes to the video include adding coffee and one less egg. White sugar has also been added and some quantities have changed. The sugar is now mixed with the dry ingredients. Video will be updated in the near future.
TOOLS USED IN THIS RECIPE
A handheld beater for whipping the cream
A silicone spatula is perfect for folding and making sure nothing is left behind.