1. Preheat the oven to 160°C (fan forced). Grease a large rectangular tin (31cm x 21cm x 4.5cm deep, base measurement) and line the base with baking paper, hanging over the two long sides.
2. Using an electric mixer, cream the butter and sugar until pale. Add the eggs one at a time, beating well after each addition. Beat in the vanilla essence.
3. Sift the flour, cornflour, baking powder and salt into the butter mixture, and add the milk. Fold together until evenly combined. If it seems too thick, add a little bit more milk. Spread into the tin and smooth the surface.
4. Bake for 40 minutes, until it springs back to a gentle touch in the centre (or until a skewer inserted into the middle comes out clean).
5. Turn out onto a wire rack to cool completely.
6. For the icing, sift the icing sugar and cocoa powder into a large bowl. Add the butter and vanilla, then pour hot water, a little at a time, onto the butter to melt it. Stir until smooth and runny.
7. Lay the cake on a sheet of baking paper, and cover the top with icing. Sprinkle with coconut. Lay a sheet of baking paper over the iced surface and quickly turn the cake over. Ice and sprinkle coconut on the other side.
8. Cut into lengths about 5cm wide. Ice the cut sides and sprinkle with coconut. Cut each length into cubes. Ice the cut sides and sprinkle with coconut. Leave to set.
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