Allow cake to stand in tin for 5 minutes before turning out onto a wire rack to cool completely.
To make the icing: Combine the cream cheese, sour cream and vanilla in the bowl of a KitchenAid Stand Mixer and beat on speed 4 until smooth. Add icing sugar and continue beating until well combined. Spread over the top and sides of the cooled cake. Garnish with extra whole walnuts on top.