Steps

• We will start with the sponge cake base we need for the bottom:

  Beat the egg whites and sugar in a stand mixer, or with electric whisks, until you obtain a dense and well-whipped mixture.

  

• For the soft cocoa base I specified the exact weights of yolks and whites because egg sizes and weights vary. To give an indicative idea, I added the approximate number next to the weight, but I still recommend using a scale.

• Gradually add the yolks to the just-whipped mixture and fold gently with a spatula from top to bottom until the mixture is homogeneous.

  

• Sift the unsweetened cocoa and add it to the mixture, folding gently with a spatula from top to bottom.

  

• If you don’t have a piping bag to spread the cocoa sponge batter on the baking paper, you can pour the batter onto the lined baking sheet and spread it with a spoon or spatula.

• Now pour the batter directly into the springform pan lined with a sheet of parchment paper. Level the batter with the back of a spoon or a spatula to obtain an even thickness. Bake in a static (conventional) oven preheated to 365°F (185°C) for about 12 minutes on the middle rack.

  

• Once baked and cooled, remove the base with the help of a knife, trim the edges and place the base back into the pan still on the parchment paper.

  

• Soak the gelatin sheets in very cold water until they have softened.

  Finely chop the dark chocolate and put it in a bowl.

  

• Bring the milk with a pinch of salt to the verge of boiling.

  Remove the gelatin from the water, squeeze it, dry it well and dissolve it in the hot milk.

  

• Pour the hot mixture of milk and dissolved gelatin over the chopped chocolate, stir well until it melts and becomes smooth and velvety. Let the mixture cool until it reaches a temperature of 109°F (43°C).

  

• Whip the cream, but not excessively, until it reaches a semi-whipped consistency.
  I recommend using fresh heavy cream (whipping cream) — it is very fluffy and has a delicate, light taste. Make sure it is unsweetened. You can find it in the refrigerated section near the milk.
  

• It is important that the cream is semi-whipped — airy and creamy but not whipped too stiff. Otherwise the mousse will be too dense and heavy and difficult to level perfectly, affecting the appearance.

  

• Add the semi-whipped cream to the chocolate mixture, folding gently from top to bottom so it doesn’t deflate.
  It is important that each mousse is prepared only when the previous layer has set and solidified, because once the mousse is ready it must be poured immediately into the mold.
  

  

• For those who want to be more precise and have a kitchen thermometer, add the cream to the chocolate base when the chocolate reaches the following temperatures: 109°F (43°C) for dark chocolate, 100°F (38°C) for milk chocolate, and 97°F (36°C) for white chocolate.

• If you don’t have a thermometer, you can judge by touch: the chocolate should feel slightly warmer than body temperature.

  Following these temperatures reduces the risk that chocolate that’s too hot will melt the whipped cream, or that chocolate that’s too cold will set the cream immediately.

  

• Pour the freshly made mousse into the pan on top of the cocoa base, level carefully and evenly. Refrigerate the mousse for at least one hour, or until well set and firm, then proceed with the second layer.

  

• Repeat similar steps as above: soak the gelatin sheets in very cold water until softened.

  Finely chop the milk chocolate and place it in a bowl. Bring the milk with a pinch of salt to the verge of boiling.

  

• Remove the gelatin from the water, squeeze it, dry it well and dissolve it in the hot milk.

  Pour the hot milk and gelatin mixture over the chopped chocolate, stir until it melts and becomes smooth and velvety. Let the mixture cool until it reaches a temperature of 100°F (38°C).

  

• Whip the cream but not excessively, until semi-whipped.

  Fold the semi-whipped cream into the chocolate mixture, gently from top to bottom so it doesn’t deflate.

• Pour the freshly made milk chocolate mousse into the pan on top of the dark mousse, level carefully and evenly. Refrigerate for at least one hour, or until the mousse is well set and firm, then proceed with the third layer.

  

• Once again, soak the gelatin sheets in very cold water until softened. Finely chop the white chocolate and place it in a bowl.

  Bring the milk with a pinch of salt to the verge of boiling.

  

• Remove the gelatin from the water, squeeze it, dry it well and dissolve it in the hot milk.

  Pour the hot milk and gelatin mixture over the chopped white chocolate, stir until it melts and becomes smooth and velvety. Let the mixture cool until it reaches a temperature of 97°F (36°C).

  

• Whip the cream but not excessively, until semi-whipped.

  Fold the whipped cream into the chocolate mixture, gently from top to bottom so it doesn’t deflate.

• Pour the freshly made white chocolate mousse into the pan on top of the milk chocolate mousse, level carefully and evenly. Refrigerate for at least 2–3 hours, until the whole cake is well set and firm.

  

• Now comes the nicest part: unmold the cake from the pan with the help of a spatula by passing it around the edge, and place it on a serving plate.

  To obtain a perfect edge I recommend leaving the cake in the freezer for 20 minutes before removing it from the pan; this way the cake edge will firm up and remain perfectly smooth when you remove the side strip of parchment paper.
   

• Now let your imagination run wild and decorate the cake with chopped hazelnuts or as you prefer. You can also decide to place Kinder cookies on top as I did for my nephew Emmanuel. Here is the finished Three Chocolate Cake