In a small saucepan, melt 2 tablespoons butter; stir in brown sugar.
Spread into an ungreased 8-in. baking dish. Arrange blueberries in a single layer over brown sugar mixture; set aside.
In a large mixing bowl, cream remaining butter; beat in sugar. Add egg and vanilla; mix well. Combine flour and baking powder; add to creamed mixture alternately with milk.
Carefully pour over blueberries. Bake at 350 degrees F (176 C) for 40-45 minutes or until a toothpick inserted near the center of cake comes out clean.
Immediately invert onto a serving platter. Cool. Serve with whipped topping if desired.
