Preheat oven to 350 degree oven. Sift the dry ingredients together. Gradually whisk in the milk and vanilla to make a thick batter. Let the batter stand for 15 minutes
Place the butter in a large shallow metal baking pan; place the pan over low heat to melt the butter. Give the rested batter a quick stir, the pour it into the pan over the melted butter. Don't worry if the batter floats around the edges of the batter.
Immediately scatter the blackberries over the batter. Bake for about 55 minutes, or until the batter is puffed and set around the blackberries. Top with ice cream or whipped cream.