Heat oven to 350 degrees. Butter a 1-quart, 5 1/2-inch-diameter souffle dish. Pour 2 tablespoons melted butter into the souffle dish. Sprinkle 3 tablespoons brown sugar over butter. Scatter 1 cup blueberries over sugar. Set aside.
In a medium bowl, whisk together flour, baking powder, and salt. In another medium bowl, whisk together remaining 2 tablespoons butter, remaining 1/3 cup brown sugar, and the egg. Whisk the milk into the egg mixture. Add flour mixture; whisk until batter is smooth.
Pour half the batter into souffle dish. Sprinkle remaining 1/2 cup blueberries over batter. Spread remaining batter over blueberries. Bake until a cake tester inserted in the middle of the cake comes out clean of batter, about 45 minutes. Immediately unmold cake, inverting it onto a serving dish. Serve warm with whipped cream if desired.