Melt the butter in a casserole.
With an electrical mixer crumble the graham biscuits and mix with the cacao powder.
Mix the melted butter and the biscuits together. Pour evenly in 6 single portion bowls. Press with a spoon on the biscuits.
Place the bowls in the fridge until later.
Melt on low heat the chocolate and let it cool down.
Mix the cold melted chocolate with the egg yellow. If it becomes compact add some liquid heavy cream.
Mix gently the melted chocolate with the rest of the liquid cream. Mix with an electrical mixer for a creamy fluffy mousse.
Pour evenly on top of the cake bottom. Replace in the fridge for minimum 2 hours.