Blueberry Upside-Down Cake: Recipe Edition

This luscious cake was the perfect Independence Day treat. I loved this dessert - the berries on top (or bottom) created a jelled, almost pudding-like topping for the moist cake. 
Fruit Topping:
6 tablespoons melted butter
2/3 cup brown sugar
2 cups fresh blueberries
6 tablespoons softened butter
1/3 cup brown sugar
2 tablespoons granulated sugar
1/2 teaspoon salt
1 egg
3/4 cup sour cream
1 teaspoon vanilla
1 cup plus 2 tablespoons flour
1/4 cup cornmeal
1 1/2 teaspoon baking powder
1/4 teaspoon baking soda
Preheat the oven to 375. 
For the topping: Pour the melted butter into a 9" round cake pan and swirl to coat the bottom. Sprinkle the brown sugar on the butter and scatter the blueberries over the brown sugar.
For the cake: In a big bowl, combine the butter, brown sugar, granulated sugar, and salt until mixed. Whisk in the egg, sour cream, and vanilla. Add the flour, cornmeal, baking powder and baking soda and mix until just combined. Spread the batter over the fruit in the pan and smooth the top. Bake until just firm and toothpick comes out with moist crumbs, about 35 minutes. Let cake cool in the pan for 45 minutes and invert on serving plate.