Remove ricotta from the fridge about 30 minutes before processing. Wash the orange hot and finely grate the rind. Mix the ricotta, sour cream, milk and orange peel in a bowl with the whisks of the mixer until smooth.
Cut the chocolate into pieces and melt in a hot water bath. Pour the chocolate bit by bit, stirring constantly, into the ricotta mass.
Spread the chocolate mousse over four glasses (each containing approx. 200 ml). Chill for about 2 hours. Decorate with orange zest and chocolate shavings if desired.