Preheat oven to 350℉. Place the butter in your baking dish, and place the baking dish in the oven as it preheats to melt the butter.
In a mixing bowl, add the whole wheat pastry flour, ⅔ cup sugar, baking powder, and salt. Stir until combined.
Whisk in the milk and vanilla extract until smooth.
Pour the batter over the melted butter in the baking dish and stir if needed. Sprinkle the fresh or frozen blackberries over the batter evenly, then sprinkle with an additional 1 tbsp granulated sugar if desired.
Bake in the preheated oven for 70-75 minutes, or until baked through. Allow to cool slightly, then serve!