Pre-heat oven to 170°C. Grease a 23cm (9in) spring-form tin. Line base with greaseproof paper. Dust inside of tin with flour, then tap out the excess.

Beat caster sugar and margarine in a mixing bowl until smooth. Beat in eggs, vanilla essence and yoghurt. Sift in flour, cocoa and bicarb. Stir until fully combined. Spoon mixture into prepared tin and level.

Bake cake for 40 - 45 mins, or until a skewer inserted into centre comes out clean. Put tin on a wire rack. Cool for 5 mins. Turn cake out onto cake rake to cool completely.

For the icing, melt marg in a saucepan. Stir in cocoa. Heat gently, stirring until combined. Remove from heat. Stir in milk and icing sugar until mixture is smooth and blended.

Spread icing evenly over top and side of cake. Arrange walnuts around top edge. Leave icing to set for 10 mins before serving.