Cake: Preheat oven to 180 degrees C. Grease three 23cm round cake tins.
In a small bowl dissolve the bicarb soda in the buttermilk; set aside.
In a large bowl cream together the butter and sugar until light and fluffy. Mix in the eggs, buttermilk mixture, vanilla, coconut, baking powder and flour. Stir until just combined. Pour batter into the prepared cake tins.
Bake in the preheated oven for 30 to 35 minutes or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.
Cream Cheese Icing: In a medium bowl combine cream cheese, butter, vanilla and icing sugar. Beat until light and fluffy. Mix in a small amount of cream to attain the desired consistency. Stir in chopped walnuts and shredded coconut.
To Assemble: Spread the cream cheese icing between the cake layers and on top and sides of cooled cake.