Begin by preheating oven to 350°F. In a medium sized bowl, whisk together the flour, polenta, baking powder, and coarse kosher salt to blend. Set aside.
To make the caramel, combine 6 Tb of sugar and 3 tablespoons water in 10-inch ovenproof skillet. Stir over medium heat until sugar dissolves.
Increase heat and boil without stirring until syrup is golden amber (not too dark).
Occasionally brush down sides of skillet with a wet pastry brush and swirl skillet, about 4 minutes. Remove skillet from heat and whisk 2 Tb butter into caramel.
Next, arrange orange slices overlapping slightly, in circles atop of caramel in the bottom of skillet.
Using an electric mixer, mix ¾ c sugar, remaining 6 Tb butter, and vanilla in another medium bowl until light and fluffy. Then add egg yolks 1 at a time, beating well after each addition.
Add reserved flour mixture in 3 additions alternating with milk in 2 additions, beating batter just until mixed.
Place mixture in bowl and clean out mixing bowl VERY well. Using clean dry beaters, beat egg whites in large bowl until soft peaks form. Add remaining 1 Tb sugar and beat until stiff.
Fold ⅓ of egg whites into batter to lighten, then fold in remaining egg whites in 2 additions.
Drop batter by large spoonfuls atop orange slices in skillet, then spread evenly.
Bake cake for about 40-45 minutes or until a toothpick comes out clean and allow to cool in skillet for about 10 minutes.
Now time to flip over. Run a small knife around the cake to loosen. Place a large plate on top of skillet, holding really well. Turn upside down and voila! Squeeze any extra orange juice on top of cake.