Hazelnut mousse cake, vanilla and chocolate

The hazelnut, vanilla and chocolate mousse cake is obviously inspired by the pasticcieria mousse cakes, which I like very much, and tries to faithfully reproduce it by always considering the instrumental limits that can be had in a home.
The preparation is not difficult, but it is a bit 'long, because it requires different preparations.

Ingredients for a 20 * 20 square pan (or round / square more or less equivalent area):

For the base:

150 gr of dark chocolate
50 ml of fresh cream
30 gr of puffed rice

For the hazelnut mousse:

130 g of hazelnut paste (to prepare it at home click here)
6 egg yolks
80 grams of sugar
3 tablespoons of water
350 ml of fresh cream
8 gr of fish glue

For the vanilla mousse:

6 egg yolks
100 gr of sugar
4 tablespoons of water
1 stick of vanilla (or a small bottle of vanilla flavoring)
350 ml of cream
8 gr of fish glue

For the cocoa ganache:

90 ml of water
75 ml of milk
100 gr of sugar
40 of bitter cocoa
4 gr of fish glue

Preparation:

Start from the base:

• Toast the rice in a non-stick pan for a few minutes, then allow to cool.
• Fully cover the pan with transparent film, including the edges.
• Melt the chocolate with the bain-marie cream, then add the puffed rice, mix well and pour on the base of the cake pan. Level well and place in the freezer.

Hazelnut mousse:

Prepare a syrup with water and sugar: as soon as the sugar dissolves and the water becomes lighter, turn off the heat.
Work the yolks with the whips and add the hot syrup to the wire.
Continue to assemble until they are light and fluffy, then add the hazelnut paste and continue working.
Soak the isinglass in cold water.
Whip the cream.
Take 2 tablespoons of whipped egg yolks and place in a saucepan with softened and squeezed gelatin.
Melt the isinglass over a low flame, mixing continuously, then put the egg yolks back together.
Add the whipped cream and mix well.
Remove the cake tin from the freezer and pour in the hazelnut mousse, level well and keep in the freezer for an hour.

Vanilla mousse:

Prepare a syrup with water, sugar and vanilla seeds (obtained from the engraved berry and emptied with a knife blade): when the sugar has melted, turn off the heat.
Whip the egg yolks with the whips and add the syrup.
Whisk until the egg yolks are light and fluffy.
Soak the isinglass in cold water for 10 minutes, then squeeze well and dissolve in a saucepan with two tablespoons of whipped egg yolks.
Combine with the remaining egg yolks.
Whip the cream and add it to the egg yolks.
Mix well.
Remove the cake from the freezer and cover with the vanilla mousse, always leveling well.
Keep at least another hour in the freezer.

Prepare the frosting:

Soak the isinglass in cold water for 10 minutes.
Meanwhile, in a saucepan put the milk, water, cocoa and sugar.
When it starts to simmer, squeeze the gelatine, add it to the other ingredients and melt it in the liquid.
Turn off the heat and let it cool for 10 minutes.
After this time, remove the cake from the freezer, pour over the icing and put in the freezer for at least 3 hours.
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