Preheat oven to 180°C. Line 20 – 22cm square baking pan with baking paper and spray with cooking spray.
Melt the butter and ½ the chocolate in a large saucepan, stirring until completely melted. Stir until chocolate is melted and smooth.
Add the sugar, cocoa, vanilla and 2 pottles of Yoplait Greek Style Blueberry yoghurt, and mix.
Add the flour and baking powder, fold in then fold in the remaining chocolate chips and the blueberries. Pour the mixture into the pan and bake 30 minutes.