Preheat oven to 180°. Grease the baking tin and line it with butter paper.
Blend cocoa with water in a small bowl.
Beat butter and sugar in a small bowl until light and fluffy. Beat in eggs, one at a time. Transfer the batter to a large bowl and stir in sifted flours, soda, buttermilk and cocoa mixture in it.
Spread mixture onto the tin. Bake about 30 minutes. Let the cake cool for about 20 minutes before turning.
Make chocolate frosting and spread it over the cake. Finally top it with blueberries.
Chocolate frosting: Melt chocolate and butter in small saucepan and stir it over low heat. Remove from heat and stir it in sifted icing sugar and water until smooth.
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