How Do You Make Chocolate Mousse Cake?

Ingredients
4 egg whites

½ cup (110 g) sugar

5 oz (140 g) hazelnuts, finely ground

2 tablespoons unsweetened cocoa powder

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9 oz (250 g) dark semi or bittersweet chocolate (60-70%)

4 egg yolks

1/3 cup (70 g) sugar

¼ cup + 1 cup (0.6 + 2.4 dl) whipping cream

Method

1. Preheat oven to 350 deg F (Gas mark 4 or 180 deg C).
   
2. Line a 9 inch (23 cm) cake tin with grease proof or other non-stick paper and grease the tin.
   
3. Whisk
   the eggs whites until stiff and in peaks by using an electric mixer,
   gradually add the sugar and whisk well each time sugar has been added.
4. Mix with finely ground hazelnuts and cocoa powder.
   
5. Bake at 350 degrees for 15 minutes or until a wooden pick inserted in center comes out clean.
   
6. Cool the cake.
   
7. Melt finely chopped dark chocolate over hot water.
   
8. Beat the egg yolks with 2 oz sugar until white.
   
9. Heat ¼ cup whipping cream to the boiling point, and whisk in beaten egg yolks.
   
10. Fold in melted chocolate. The mixture will immediately thicken. Cool to approx. 100 deg F / 38 deg C.
    
11. Whisk 1 cup of whipping cream. Fold with the egg and chocolate mixture, one tablespoon at a time until the
    filling becomes soft, then the remaining part of the cream.
    
12. Transfer to the cake tin, and place in the refrigerator for several hours.
    

If served cold the filling appears more like chocolate truffle than mousse.  
If you prefer a more sweet flavor of the filling you can use semisweet chocolate (40-45%) instead of the recommended type.
Variations
For decoration - make chocolate panels to cover around outside of cake.  To make chocolate panels:  
Melt 8 oz (250 g) bittersweet chocolate. Spread onto a piece of baking paper. Allow to become mat
dry. Mark panels height of cake x 2 inches (5 cm) by scoring with a knife.  When set a little more,
break off panel pieces and stick onto outside of cake by using a little mousse as a 'glue'.  
Overlap each piece little.  Decorate top with piped mousse, chocolate dipped strawberries, etc.  
Tie a ribbon around circumference of cake