Make the base: mix together the muesli, melted butter, and sugar, and press evenly over the bottom of a 20cm loose-bottomed or springform tin. Chill.
Meanwhile, put the chocolate into a small heatproof bowl over a pan of hot, gently simmering water. Heat gently to melt the chocolate, stirring occasionally. Leave to cool.
Put the measured water into a heatproof bowl and sprinkle the gelatine over the top. Leave for 10 minutes until spongy. Stand the bowl in a pan of hot water and heat gently until the gelatine has dissolved.
Beat the cheese until smooth. Add the egg yolks and sugar and beat until blended. Stir in the soured cream, melted chocolate, chocolate chips, and gelatine. Mix well.
In a separate bowl, whisk the egg whites until stiff but not dry. Fold carefully into the chocolate mixture until evenly mixed. Pour on to the muesli base and chill until set.
Use a knife to loosen the side of the cheesecake from the tin, then remove the cheesecake. Slide on to a serving plate. Pipe rosettes of whipped cream on top and decorate with chocolate curls or caraque.