Place 1 tablespoon lemon juice into a full sized measuring cup, fill to top with milk, set aside for 5 minutes. This will create a buttermilk substitute.

Preheat oven to 180oC. Lightly grease a 6cm-deep, 20cm round cake pan.

Sift flour, bicarbonate of soda and cocoa powder into a large bowl. Stir in sugar.

Combine buttermilk substitute with eggs, melted butter and vanilla. Make a well in dry mixture and pour in the wet mixture.

Using an electric mixer, beat on low speed to combine, then on high until mixture is thick and creamy.

Pour mixture into prepared pan. Bake for 40 to 45 minutes or until skewer inserted into centre comes out clean. Allow to cool in pan for 5 minutes. Turn onto wire rack to cool completely.

Sift icing sugar and cocoa powder in a bowl. Add butter and 1 tablespoon boiling water. Stir until smooth. Spread over cake. Allow icing to set then serve.