Alternating dry ingredients and melted butter, mix in 1/3 of the dry ingredients into the wet, followed by ½ of the melted butter. Repeat until all the ingredients are just combined.
Fold in most of the mixed berries. Reserve ½ cup of the fresh or frozen berries for topping. Evenly spread the buttery berry cake batter into the prepared pan. Top with the reserved berries and more sugar if you like.