Ingredients:

Chocolate Sponge

• 6 Eggs (yolk and white separated)
• 225g Sugar
• 125g All Purpose Flour (sifted)
• 25g Cocoa Powder (sifted)
• 7g Baking Powder (sifted)
• 2g Salt
• 25g Unsalted Butter
• 1tbsp White Wine Vinegar
• 1tbsp Vanilla Extract

Dark Chocolate Mousse

• 125g Dark Chocolate (minimum 70%, cut into small pieces)
• 50g Unsalted Butter (cut into small pieces)
• 6 Egg Yolks
• 125ml Strong Coffee
• 100g Sugar
• 250ml Heavy Cream (well chilled)
• 50ml Alcohol (Rum, Amaretto, Whisky, Bourbon or Brandy)
• 25g Cocoa Powder (sifted)

Toffee Sauce

• 100g Sugar
• 100ml Water
• 50ml Heavy Cream
• 25g Unsalted Butter
• 1tbsp Alcohol (Rum, Amaretto, Whisky, Bourbon or Brandy)

Chestnuts in Coffee Syrup

• 100g Whole Chestnuts (roasted, and peeled)
• 200ml Strong Coffee
• 200g Sugar

Instructions:

Chocolate Sponge

1. Preheat the oven to 350ºF.
2. Grease a sheet pan (18inch x 13inch) all over with butter, and line the base of the sheet pan with non-stick parchment paper and apply another coating of butter on top.
3. Pour your flour, cocoa powder and baking powder into a bowl and stir until well combined.  Set aside.
4. In a bowl of a stand mixer, add your egg whites, salt and vinegar and whisk under medium high speed for 1-2mins until foamy.  Add half your sugar, and continue whisking for approximately 5-6mins on high speed until you’ve reached stiff peaks and the meringue becomes glossy.  Using a rubber spatula, remove your meringue mixture from the stand mixer bowl and set it aside in another bowl.
5. Add your egg yolks, sugar and vanilla into your stand mixer bowl and whisk under medium speed for 2-3mins until thoroughly combined and foamy.
6. Add all your dry ingredients into your egg yolk mixture a third at a time.  Whisk under low speed until just combined, and scrape the bottom and sides of the bowl with a rubber spatula before each new addition of dry ingredients.
7. Add 1/3 of your meringue into your chocolate and egg mixture and fold it in until well combined, before gently folding in the remaining 2/3 of your meringue.
8. Pour your cake mixture into your baking sheet and spread it evenly across the surface.  Place it in the oven and bake for approximately 20mins until cooked.  When ready, remove and set aside.

Dark Chocolate Mousse

1. Place your egg yolks and alcohol into a bowl of a stand mixer and whisk for 2-3mins under medium speed until light and foamy.
2. Place your coffee and sugar into a pan and cook under medium high heat until it reaches 250ºF and becomes a thick syrup.
3. Begin whisking your egg yolk mixture again under low speed and slowly pour all your coffee sugar syrup into your egg yolks in a thin steady stream.  After you’ve added all your coffee syrup, turn up the speed of your stand mixer to high and continue whisking for approximately 5-6mins until the mixture has quadrupled in volume and has cooled down to room temperature.  Remove and set aside in another bowl.
4. Place your chocolate and butter into a bowl and cook under a double boiler until both the chocolate and butter are completely melted.
5. Add your heavy cream to the bowl of your stand mixer and whisk under medium speed for 2-3mins until stiff peaks.
6. Stir in your melted chocolate mixture into your whipped cream until well combined.  Than gently fold in your coffee egg mixture until thoroughly combined and homogeneous.

Toffee Sauce

1. Add your sugar and water into a pan and cook under medium high heat until it turns into a caramel.  The caramel should be a dark amber brown.
2. Immediately add your butter and heavy cream and stir until well combined.  Cook for another minute before turning off the heat and finishing your toffee sauce with alcohol.

Chestnuts in Coffee Syrup

1. Place your coffee and sugar into a pan and cook under medium high heat until the mixture comes to a boil, and cook for an additional 5mins.
2. Reduce your heat to low and add your chestnuts to your coffee syrup and cook gently for 20mins.

Assemble

1. Using a cake pan (9 inches in diameter, with a removable base) as a guide, cut out a circle of the chocolate sponge cake and line the base of the cake pan.
2. Pour your toffee sauce all over the top of your chocolate sponge base and spread it around till there’s a even coating.
3. Pour all your dark chocolate mousse mixture into the cake pan and using a palette knife smooth over the surface of your mousse mixture.
4. Dust the top with cocoa powder and place your cake into the refrigerator for 4-6hrs to chill and firm up.
5. Run a thin shallow knife around the edge of the cake pan, before carefully removing the ring around your cake pan and lifting the cake from the base of the cake pan and onto a dish.
6. Garnish your Dark Chocolate and Toffee Gateau with your Chestnuts and Coffee Syrup around the edges as well as some grated dark chocolate.

 

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Notes:

• What I like most about this dish is that while the presentation is somewhat understated, the amount of flavours it delivers is extraordinary.  The coffee notes in the background really gives you a lot of depth and complexity to this cake.
• Certain ingredients have a natural affinity with one another and just seem to work together to produce a end product greater than the sum of its parts.  In this case, the dark chocolate, coffee, vanilla and cream really come together to produce an amazing and delicious end product far greater than it’s individual components! 
• I know on first appearance, this recipe is a bit disorienting but it’s really not that difficult.  You can break it up in stages to help manage the workload.  For example, you can omit the chestnuts and prepare the chocolate sponge base and toffee sauce the day before and set them aside.  Than do the dark chocolate mousse and quickly assemble the cake without any issues. 
• A good tip when beating eggs, is to use room temperature eggs versus eggs that are straight out of the fridge and stone cold.  The reason being that eggs that are room temperature are easier to whip and produce more volume.
• When folding in the meringue into the cake mixture, the first 1/3 that you add you can be aggressive and vigorously stir it into the cake mixture to lighten it before gently adding and folding in the remaining 2/3 of the meringue.  The reason being you want to ensure the cake mixture is light and uniform enough to take the remaining 2/3 of the meringue without the risk of deflating it.   
• In my opinion, the best alcohol to use in this dish is Amaretto.  As the almond flavour within the alcohol complements the flavour of the chocolate better than the use of other alcohols.  However, in saying that it’s not the end of the world if you use another type of alcohol you happen to have at home or is available locally.  For example, if you happen to be from States and there’s a particularly good quality whisky or bourbon available in your area than by all means you should use that instead.  
• This cake is better after it’s been given a day or so to set and mature in the fridge, but will keep just fine for 5-7days without any issues.  Though I should warn you that if you leave it there that long, your fridge will smell like coffee and chocolate.  Whether that’s a good or bad thing, that’s really up to you lol!