Preheat the oven to 180°C and grease a 20cm springform tin with butter.
Beat eggs and sugar in a heatproof bowl over simmering water for five minutes.
Sieve in the flour and beat until well combined.
Bake the batter in the tin for 8-10 minutes until golden then cool completely.
Simmer sugar, blackcurrants, and liqueur in a pan for 5 minutes.
Dissolve gelatine leaves in 22.5ml of hot water for 10 minutes.
Puree the fruit in a processor then blend with gelatine, cream, and yoghurt.
Fold the whisked egg whites into the mousse mixture until just combined.
Spread mousse over the cake base and chill overnight in the refrigerator.
Serve the cake with chocolate sauce and extra whipped cream.