Begin by preheating your oven to 350°F and preparing two round cake pans (either 8x1½ or 9x1½ inches) by generously greasing the bottoms and sides with shortening and lightly flouring them.
In a large bowl, combine 1 (18 ounce) box white cake mix, 3/4 cup water, 1/3 cup vegetable oil, 3 egg whites, and 1 (6 ounce) container of original Yoplait strawberry yogurt. Using an electric mixer, beat the mixture on low speed for 30 seconds, then increase to medium speed and beat for 2 minutes until well combined.
Divide the batter evenly between the prepared pans and bake for 23 to 28 minutes for 9-inch rounds or 27 to 32 minutes for 8-inch rounds, until a toothpick inserted in the center comes out clean.
Once baked, allow the cakes to cool in the pans for 10 minutes before running a knife around the sides to loosen them. Turn the cakes out onto a wire rack and let them cool completely, approximately 1 hour.
While cooling, prepare your strawberries by cutting about 10 of them into ¼-inch slices for the filling and halving the remaining berries for the top garnish.
To assemble, place one cake layer on a serving plate and spread 1/3 cup of whipped vanilla frosting over it, leaving a ¼-inch border around the edge. Arrange the sliced strawberries on top of the frosting and place the second cake layer on top.
Frost the sides and top of the assembled cake with the remaining frosting, then arrange the halved strawberries on top as a decorative garnish. Store the finished cake loosely covered in the refrigerator until ready to serve.