A chocolate mousse cake is chocolate sponge and airy chocolate and cream mousse under a ganache. The usual failures: flecks of chocolate in the mousse, a mousse that will not set, and a cake that slumps when cut. The first is chocolate temperature; the others are time in the cold.
What it is made of
Top to bottom:
- Dark ganache 0.5 cm
- Chocolate mousse ×2 · 2.0 cm
A soft, airy layer
- Chocolate sponge ×2 · 2.5 cm
Two sponges, two layers of mousse, ganache.
The mousse is chocolate, whipped cream, eggs and sugar. It sets only from the cocoa butter in the chocolate as it chills; there is no gelatine in this version. So it is softer than a gelatine mousse cake and needs to set fully in the cold.
Order of work
Sponges (the day before): chocolate sponge with baking soda, because natural cocoa is acidic. Eggs and butter at room temperature. Mix dry and wet separately, combine in 10 to 15 seconds. Fill the tin to two thirds. Bake at about 180 °C until a skewer is dry. 10 to 15 minutes in the tin, then a rack. Do not wrap warm. Chill 4 to 10 hours, slice with a wire.
Mousse:
- Melt the chocolate and cool it to 35 to 40 °C: hot chocolate melts the cream, too cool and it sets in flecks.
- Eggs: warm the yolks with sugar over a water bath until thick, about 70 °C, or use pasteurised.
- Whip cold cream (33 to 35%) to soft peaks.
- Fold the chocolate base into the cream in portions, from the bottom up. Over-whipped cream cannot be fixed; it goes grainy.
Assembly (in a ring with an acetate band):
- Bottom sponge, a 2 cm layer of mousse.
- Second sponge, mousse.
- Freezer for 1 to 2 hours, or fridge 6 to 8 hours: the cake must set fully.
- Ganache: cream (30% or more) at 80 to 90 °C, chocolate, cold butter. Pour at 34 to 36 °C onto the chilled cake.
Slice with a warm dry knife, wiping it after each cut.
What can go wrong
- Flecks in the mousse. The chocolate was too cold.
- Runny mousse. The chocolate was hot or the cream under-whipped.
- Mousse did not set. Too little time in the cold.
- Cracked ganache. Poured onto a cake that was too cold.
The verdict
Chocolate temperature and setting time decide the mousse. Do not rush the cold.