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1
Position rack in center of oven and preheat to 350u00b0F. Combine 6 tablespoons sugar and 3 tablespoons water in 10-inch-diameter ovenproof skillet with 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 sides of skillet with wet pastry brush and swirling skillet, about 4 minutes. Remove skillet from heat and whisk 2 tablespoons butter into caramel. Set aside.
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2
Cut off both rounded ends of each orange so that ends are even and flat. Using sharp knife, cut oranges into 1/16- to 1/8-inch-thick rounds. Remove and discard any seeds. Arrange orange slices, overlapping slightly, in concentric circles atop caramel in bottom of skillet.
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3
Whisk flour, polenta, baking powder, and coarse kosher salt in medium bowl to blend. Using electric mixer, beat 3/4 cup sugar, remaining 6 tablespoons room-temperature butter, and vanilla in another medium bowl until light and fluffy. Add egg yolks 1 at a time, beating well after each addition. Add flour mixture in 3 additions alternately with milk in 2 additions, beating batter just until incorporated.
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4
Using clean dry beaters, beat egg whites in 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 2 additions. Drop batter by large spoonfuls atop orange slices in skillet, then spread evenly.
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5
Bake cake until tester inserted into center comes out clean, about 45 minutes. Cool cake in skillet 10 minutes. Run small knife around cake to loosen. Place platter atop skillet. Using oven mitts, hold platter and skillet firmly together and invert, allowing cake to settle onto platter. Rearrange any orange slices that may have become dislodged. Cool cake completely at room temperature.
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6
Using electric mixer, beat chilled creme fraiche and 2 tablespoons sugar in medium bowl until mixture thickens.
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7
Cut cake into wedges and serve with dollop of whipped creme fraiche.
