1.Preheat oven 180°C (160°C fan forced). Spray a 19cm x 30cm (7½-inch x 12-inch) rectangular pan with cooking oil, use baking paper to line the pan.
2.Stir the cocoa and the water in a small bowl with a small spoon until the mixture is smooth. Leave it to cool down.
3.Combine butter and sugar in a small bowl, beat with an electric mixer until the mixture is light and fluffy. Add one egg, beat into the mixture. Do this, one at a time, with the other two eggs. Transfer the mixture to a large bowl. Add half the sifted flours, bicarbonate of soda and buttermilk to mixture, stir them in with a wooden spoon. Add the rest of the sifted flours, bicarbonate of soda and buttermilk, stir them in. Stir in the cooled cocoa mixture.
4.Put the mixture into the pan, spread it evenly with a spatula to cover the base of the pan. Put the pan in the oven and bake about 30 minutes. Remove from oven, rest for 20 minutes. Turn the cake from the pan and put it on a wire rack, top-side up, leave it to cool down.
5.Make the chocolate icing. Put chocolate and butter in a small saucepan. On low heat stir with a spoon until smooth. Take pan off heat. Add the sifted icing sugar and water, stir until smooth.
6.Spread the icing over the top and sides of the cold cake with a metal spatula, put the blueberries on top. Cut the cake into 20 squares.
Stir the cooled cocoa and water mixture into the cake batter. Spread the cake mixture into the greased and lined cake pan. Use a spatula to smooth the top.Note