Once cooled, individual Pineapple Upside-Down Cakes can be immediately turned out onto pretty dessert plates for serving.
2. Spray four 4-inch-wide shallow ramekins with nonstick cooking spray. Sprinkle the bottom of each with 1/4 teaspoon sugar, place a slice of pineapple in each ramekin, and set aside.
3. Sift together the flour, remaining sugar, baking powder, and salt into a large mixing bowl. Make a well in the center of the dry ingredients and add the oil, pineapple juice concentrate, and vanilla. Set aside.
4. In another large mixing bowl, beat the egg whites until they are white and foamy, about 1 minute. Add the confectioners' sugar and continue beating until stiff but not dry, about 2 minutes. Scrape off the beaters and, without washing them, place in the flour mixture and beat at low speed just until blended. Gently fold the flour mixture into the egg whites.
5. Pour one quarter of the batter into each ramekin and bake for 25 minutes, or until a cake tester inserted in the center comes out clean.
6. Cool the cakes on a wire rack. With a sharp knife, loosen the sides of the cakes. Invert onto the rack, lift off the ramekins, and cool. Serve warm.