Preheat oven to 350ºF. Spray 12 in. round pan with vegetable pan spray.
In large bowl, combine 1 package cake mix, 1/2 cup sour cream, 4 egg whites and 2 tablespoons oil with electric mixer; beat on high 2-3 minutes. Reduce mixer speed to low; gradually add 1 package pureed strawberries. Continue mixing until well blended, about 2 minutes. Pour batter into prepared pan.
Bake 35-40 minutes or until toothpick inserted in center comes out clean. Cool in pan on cooling rack 12-15 minutes. Remove from pan and cool completely. Repeat with remaining cake mix, sour cream, egg whites, oil and pureed strawberries. Bake 35-40 minutes. Cool in pan on cooling rack 12-15 minutes. Remove from pan and cool completely.
Stack cooled cake layers, using strawberry preserves for filling. Spatula ice cake with buttercream icing. Top with sliced strawberries.