Rhubarb Upside-Down Cake

Preheat oven to 350 degrees F. In 12-inch nonstick skillet with oven-safe handle (if not oven-safe, wrap handle in double thickness of foil for baking in oven later), melt 3 tablespoons butter on medium, swirling to coat side of pan. Sprinkle brown sugar over melted butter and continue to cook 1 to 2 minutes or until sugar begins to melt, stirring constantly. Remove skillet from heat. Trim tough stem ends from rhubarb stalks. Place rhubarb stalks in skillet in parallel rows across top of sugar mixture, cutting and piecing together stalks as necessary to almost completely cover bottom of skillet. On waxed paper, combine flour, baking powder, and salt. In small bowl, with mixer on medium speed, beat egg whites until foamy. Increase speed to medium-high, and beat until soft peaks form. Gradually beat in 1/4 cup granulated sugar, and beat until whites stand in stiff peaks when beaters are lifted. In large bowl, with mixer on low speed, beat remaining 1/2 cup butter and remaining 3/4 cup granulated sugar until blended. Increase speed to medium; beat 3 minutes or until light and fluffy and a pale lemon color, occasionally scraping bowl with rubber spatula. Beat in egg yolks, 1 at a time, and vanilla until well blended. Reduce speed to low. Beat in flour mixture alternately with milk just until blended, occasionally scraping bowl. Stir in 1 large spoonful beaten whites to loosen batter slightly. Then, with rubber spatula, fold in remaining whites, half at a time, just until blended. Transfer batter to skillet and spread evenly. Bake 38 to 40 minutes or until toothpick inserted in center of cake comes out clean. Cool cake in skillet on wire rack 10 minutes. Invert cake onto platter and remove skillet; cool slightly, about 30 minutes, to serve warm, or cool completely, about 2 hours, to serve at room temperature.