To make the cake

Position rack in center of oven and preheat to 350° F.

Combine 6 tablespoons sugar and water in a 10-inch-diameter ovenproof skillet with an 8-inch-diameter bottom and 2 1/2-inch-high sides. Stir over medium heat until sugar dissolves. Increase heat and boil without stirring until syrup is golden amber (not dark amber), occasionally brushing down the sides of the skillet, about 4 minutes. Remove skillet from the heat and whisk 2 tablespoons butter into caramel. Set aside.

Cut off both rounded ends of each orange so that the ends are even and flat. Using a sharp knife, cut oranges into 1/8-inch-thick rounds. Remove and discard any seeds. Arrange orange slices, overlapping slightly, in concentric circles atop caramel in the bottom of the skillet.

Whisk flour, polenta, baking powder and coarse salt in a bowl to blend. Using an electric mixer, beat 3/4 cup sugar, remaining 6 tablespoons room temperature butter and vanilla in another bowl until light and fluffy. Add egg yolks one at a time, beating well after each addition. Add flour mixture in three additions alternately with milk in two additions, beating batter just until incorporated.

Using clean, dry beaters, beat egg whites in a large bowl until soft peaks form. Add remaining 1 tablespoon sugar and beat until stiff but not dry. Fold 1/3 of egg whites into batter to lighten, then fold in remaining egg whites in two additions. Drop batter by large spoonfuls atop orange slices in the skillet, then spread evenly.

Bake cake until a tester inserted into center comes out clean, about 45 minutes. Cool cake in the skillet for 10 minutes. Run a small knife around the cake to loosen. Place a platter atop the skillet. Using oven mitts, hold the platter and skillet firmly together and invert, allowing cake to settle onto platter. Rearrange any orange slices that may have come off. Cool cake completely at room temperature.

For whipped crème fraîche

Using an electric mixer or beaters, beat chilled crème fraîche and 2 tablespoons sugar in a bowl until the mixture thickens. If using whipping cream, beat with sugar until soft peaks form.

Cut cake into wedges and serve with a dollop of whipped crème fraîche.

Special notes:

PCC carries crème fraîche from Bellwether Farms. Look for it in the cheese department.