Step 1

Preheat your oven to 350°F / 180°C and grease two 8-inch cake pans.

Step 2

In a large bowl, sift the dry ingredients including the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk everything together.

Step 3

In a separate bowl, combine all the wet ingredients, except the hot water, this includes the eggs, milk, sour cream, vegetable oil and, vanilla. Whisk together lightly until it makes a homogenous mixture.

Step 4

Pour the wet ingredients into the dry and whisk until everything is combined and there are no lumps.

Step 5

In a small bowl or a glass, combine the hot water with the espresso powder and mix until completely dissolved. Pour into the cake batter and mix until combined.

Step 6

Divide the batter evenly between your two cake pans and bake for 35 minutes, or until a skewer inserted comes out clean.

Step 7

Let the cakes cool completely before removing them from the pans. Level off the tops of the cakes with a serrated knife or cake leveler, so they are completely flat. You may also want to cut the layers down to a thinner size if you want a thinner layer of cake.

Step 8

Place a cake ring on top of a cake board or serving plate and place one of the layers of cake in the bottom. Set aside until the mousse is ready.

Step 9

Chop the dark chocolate finely and set aside in a medium bowl. Do the same with the white chocolate.

Step 10

Put egg yolks in a bowl and whisk until they become slightly thick and pale.

Step 11

In a saucepan over medium heat, warm half of the cream and the vanilla. Once it starts to feel warm, drizzle a little of the cream into the egg yolks and whisk it in. Keep drizzling a little more, whisking the whole time, until all the cream is added. Then pour the entire mixture back into the saucepan and return to the heat.

Step 12

Heat the mixture, whisking constantly (but not vigorously - just enough to keep it moving and avoid burning) until it begins to thicken.

Step 13

Remove from the heat and put half of the mixture into the bowl with dark chocolate and half in with the white chocolate. Mix each bowl until the chocolate has completely melted.

Step 14

In a separate bowl, whip up the remaining heavy cream until stiff peaks form. Split the whipped cream evenly between the two chocolate mousse mixtures and fold it in gently until completely combined.

Step 15

Pour the dark chocolate mousse on top of the cake layer in your dessert ring and smooth it out with a spatula.

Step 16

Pour the white chocolate mousse on top and smooth it out in the same way. The dark chocolate will be a lot firmer than the white chocolate, so you should be able to pour the white chocolate mousse on top without them mixing together. If your dark chocolate is still very liquid, put it in the fridge for a few minutes after pouring it onto the cake before adding the white chocolate layer.

Step 17

Put the entire cake in the fridge for 30 minutes. After 30 minutes, add the second cake layer on top, then return to the fridge to set for at least 4 hours.

Step 18

Once the cake has been chilling for at least 4 hours, remove it from the fridge and release it from the cake ring, return to the fridge while you make your ganache.

Step 19

Chop your chocolate finely and place it in a bowl.

Step 20

Heat the cream on medium heat, just until scalding (tiny bubbles just starting to form). Don't let it boil. Remove from the heat and pour the cream over the chocolate. Let it sit for a minute.

Step 21

Mix the cream and the chocolate together until completely melted, glossy and smooth. Let it cool until it is a spreadable consistency, but not completely set (about 30 minutes depending on conditions). It should be thick enough that it won't just run off the side of the cake, but thin enough to spread out into a nice smooth layer on top of the cake.

Step 22

Spread the ganache on top of the cake and use a spatula or knife to spread it out to the edges.

Step 23

Decorate with sprinkles or chocolate shavings.