Cake: Preheat the oven to 190 degrees C. Grease and line two 20cm round cake tins with baking paper.
Beat butter and caster sugar in a small bowl with an electric mixer until light and fluffy. Beat in the eggs, one at a time. Fold in the sifted flour.
Divide the mixture evenly between the two prepared cake tins. Bake for 25 to 30 minutes or until the centre of the cakes spring back when lightly touched. Stand in tin 5 minutes before turning out onto wire racks to cool.
Icing: Mix together the icing sugar and the butter until light and fluffy. Dissolve the cocoa powder in the boiling water. Stir into the sugar mixture with the vanilla.
To Assemble: Cut each cake in half horizontally. Sandwich the layers together with a quarter of the icing then place on a serving plate.
Spread 3/4 of the remaining icing over top and sides of cake. Press chopped nuts onto the sides of the cake. Arrange pear halves on the top. Decorate with swirls of the remaining icing and walnut halves.