Orange Upside-Down Cake

1. Preheat the oven to 350°F.
2. Wash the orange. Cut off and discard the ends. Cut the orange crosswise into slices about 1/4 inch thick. Remove any seeds. Put in the skillet, add 1/4 cup water,and sprinkle with sugar. Bring the mixture to a boil over high heat, gently stirring to keep the fruit from sticking. Cook until the liquid is almost gone and the orange slices are caramelized. With a fork, move the orange slices into an attractive pattern in the bottom of the skillet.
3. In a small saucepan, heat the milk until warm but not hot. Remove from heat and add 1 teaspoon of the sugar and the yeast. Stir until the yeast is dissolved.
Set aside.
4. In a large bowl, beat together the 2 egg yolks, the remaining sugar, and the margarine until pale yellow and thick. Add the orange zest and vanilla; mix well. Sift the cornmeal into the egg mixture, stirring constantly. Stir in the yeast mixture,which should now be frothy.
5. In a clean bowl, beat all 4 egg whites until they form stiff peaks. Stir a third of the beaten whites into the cornmeal mixture. Carefully fold in the remaining whites. Spoon the batter in dollops over the orange slices in the skillet. Smooth the top.
6. Bake for 25 to 30 minutes, until a cake tester inserted in the middle comes out clean. Run a knife between the sides of the skillet and the cake. Cool on a rack for 10 minutes. Hold a flat serving plate over the skillet and invert the cake. Serve while still warm.