In a saucepan, add butter and blueberries and simmer over low heat until butter is melted. Remove from heat. In a separate bowl, mix eggs, sugar, and vanilla. Add a spoonful of the blueberry mixture to the egg mixture to temper it. (you dont want your egg to scramble)
Now add your tempered egg mixture to the saucepan of blueberries ans mix well.
Add in cocoa, flour, salt, and baking powder. Mix until just combined.
Bake for 25-35 minutes, until a toothpick comes out almost clean. (try not to overtake)