Triple Chocolate Mousse Birthday Cake

Here is the second cake I made this weekend.  This is the birthday cake for the boyfriend's sister.  Their birthdays are only 3 days apart so they had a joint party today.  She was a big fan of the triple chocolate mousse cakes I made a couple of months back, having eaten the one meant for her mother, and requested a birthday cake version.

I had to estimate how much I had to increase the amounts by.  I ended up quadrupling the recipe for the cake and that worked fine.  But I only doubled the amount of mousse and it was far too much.  I think I only needed the original amount.  Of course, part of me thinks I should have covered the entire top with the mousse and written the inscription in dark chocolate but never mind.  It still looked pretty.  I also didn't think I could pull off the chocolate shell, since I failed last time, so substituted it with a ring of chocolate fingers. 

The inscription on top is a reference to the game Portal and was specifically requested.

Recipe - adapted from Baking Heaven magazine

Cake:

• 240g/8oz unsalted butter
• 240g/8oz caster sugar
• 4 eggs
• 180g/7oz self-raising flour
• 60g/2oz cocoa

White Chocolate Mousse:

• 150g/5.5oz white chocolate
• 225ml/7.5fl.oz double cream

Chocolate Buttercream:

• 30g/1oz unsalted butter 
• 60g/2oz icing sugar
• 60g/2oz dark chocolate 
• 2 x 125g boxes of Cadbury chocolate fingers

Other: 

• About 20g/0.7oz white chocolate for piping a message/decoration (optional)
• 50cm/20inch length of ribbon 

1. Preheat the oven to 180°C/350°F/Gas Mark 4 and grease two 20cm/8inch cake pans.
2. Put the butter and sugar in a bowl and beat with a whisk until pale and creamy.
3. Add the eggs one at a time and mix well after each addition.
4. Sift the flour and cocoa together into a separate bowl and then add to the sugar mixture.  Fold in.
5. Spoon into the cake pans, trying to get an equal amount in each and level out the tops.
6. Bake for about 20 minutes or until a skewer inserted into the centre comes out clean.
7. Once cooked, leave to cool on a rack.
8. Make the mousse by melting the white chocolate and leaving to cool.
9. Whip the cream and then fold into the cooled white chocolate.
10. Spread 2/3 of the mousse over the top of one of the cake layers then place the other layer on top.  Wipe away any mousse that spills over the edges.
11. Make up the chocolate buttercream by melting the chocolate in a bowl placed over a pan of hot water then leaving to cool.
12. Mix together the butter and icing sugar, then stir in most of the melted chocolate, reserving some for drizzling.
13. To make the chocolate finger ring, spread buttercream over the back of a chocolate finger and then press against the cake.  Repeat, placing the chocolate fingers right up against one another until you have surrounded the entire cake.  Tie with the ribbon to help keep secure until it sets.
14. Melt the white chocolate for piping in a bowl placed over a pan of hot water.
15. Put into a piping bag fitted with a small round tipped nozzle and pipe your message or decoration on top of the cake.
16. Put the remaining mousse into a piping bag an cut off the end.  Pipe the mousse around the outer edge of the cake.  The chocolate fingers should stop it spilling over the edge.
17. Drizzle the remaining dark chocolate over the mousse.
18. Put in the fridge to set for about 3 hours.