In a large mixing bowl, mix the flour, sugar, cocoa powder, baking powder, baking soda, salt, and espresso powder (if using). Whisk to combine.

In another bowl, combine the eggs, vegetable oil, buttermilk, sour cream, and the vanilla extract. Whisk until completely incorporated.

Pour the wet ingredients over the dry ingredients and whisk until combined. When the batter is incorporated, add the hot coffee and mix to combine.

Whisk until the batter is smooth. The chocolate cake batter will be very thin and liquidy.

I like to transfer it to a bowl with a spout to help distribute the cake batter between the cake pans, which helps with such a runny batter.

Bake the cakes in the oven for about 30 to 35 minutes, until a toothpick inserted into the cake comes off clean.

Let the cakes cool down for 15 minutes in the pan.

Get all the ingredients separated and measured out, but keep the cream in the fridge so it remains cold.

Place the chocolate in a large bowl. Microwave for 15 to 30 second intervals, stirring in between, until the chocolate melts completely.

Allow for the chocolate to cool down for about 5 minutes.

Add the yolks to the chocolate one at a time, whisking until incorporated. The mixture will get very thick and might get hard to whisk so you may have to use a spatula instead.

Place the water in a small bowl. Sprinkle the gelatin powder over the water and let it sit for a couple of minutes.

Meanwhile heat up 1/2 cup of the heavy cream in the microwave until hot and steaming.

Pour the bloomed gelatin in the hot cream and stir to combine until the gelatin has dissolved with the hot cream.

Pour the hot cream mixture over the chocolate/yolk mixture just a little at a time, stirring with a spatula until incorporated before adding the next little bit.

Set the chocolate/yolk/gelatin mixture aside.

Place the remaining 1 cup of cold heavy cream in a large bowl or in the bowl of a stand mixer. Whip on medium-high speed for a few minutes with the whisk attachment or with a hand mixer until stiff peaks form. Set it aside.

Place the egg whites and cream of tartar in a bowl, or in the bowl of a stand mixer. With the whisk attachment, whip until it becomes whitened in color and foamy. Then start adding the sugar while whisking on medium speed. Gradually add the sugar in until it’s all been added and continue to whip until stiff peaks form.

Add half of the whipped cream to the chocolate/gelatin mixture and fold to combine.

Add the remaining half and fold again.

Add half of the whipped whites and fold gently.

Add the remaining whipped whites and fold until incorporated.

The mousse will be very runny, it’s ok, it will firm up in the fridge once you assemble the cake.

You will need a large (9-inch, or adjustable) pastry ring and acetate to assemble the cake. If you don’t have a pastry ring, you can use a springform pan, but then I recommend halving the mousse recipe because you will have too much mousse and won’t be possible to form such a tall cake in the springform pan. Alternatively, you can make the full mousse recipe and divide whatever mousse doesn’t fit in the pan between ramekins and serve them like that.

The acetate is needed for the recipe to work otherwise the mousse will leak out.

Place a cake layer on the bottom of the pastry ring, or in the bottom of the springform pan with the acetate around it. You will want to leave a bit more loose at the top part so the top cake layer can slide in easily.

Pour the mousse in, on top of the cake. Top with another cake layer.

Place it in the fridge overnight, or at least 8 hours.

To make the ganache place the chopped chocolate in a large bowl.

Heat up the heavy cream and corn syrup until steaming.

Pour the heavy cream and corn syrup over the chopped chocolate.

Let it sit for one minute. Whisk until the chocolate has melted entirely.

If the chocolate hasn’t melted, microwave for 10 to 15 second intervals, stirring in between until it melts, be careful not to overheat, or it can cause the ganache to split.

Let the ganache sit at room temperature for about 1 hour until it thickens.

Remove the pastry ring from around the cake. Carefully peel off the acetate.

Spread the ganache over the cake. Decorate with berries and chocolate shavings if desired.