Preheat the oven to 180C/350F/Gas 4.
In a bowl, mix together the sugar, self-raising flour, softened butter and cocoa powder. Add the egg and enough milk to make a smooth thick mixture.
Grease and line a 20cm/8in baking tin and spoon in the chocolate mixture. Scatter the blueberries over the top and place in the oven. Bake for 18-20 minutes, or until just cooked but still soft in the middle. Cut the cake into squares.
Serve warm, dusted with icing sugar and drizzled with double cream.