Heat oven to 350 deg. Lightly grease 13x9 inch pan. Line with waxed paper.;grease and flour. In a large bowl,combine cake mix,water,sour cream and egg whites;beat at low speed until moistened. Beat 2 minutes at hight speed. Pour batter into prepared pan . Bake for 23 to 30 minutes. Cool on wire rack 15 minutes. Remove from pan;peel off waxed paper.
In a medium sauce pan,combine mashed strawberries,sugar,cornstarch and water. Cook over medium heat until mixture thickens and boils,stirring constantly. Boil 1 minute. Cool;cover and refrigerate until well chilled.
To assemble cake,slice cake in half lengthwise. PLace on half,top side down,on serving plate,spread with strawberry filling. Place remaining half top side up, on filling.
In a large bowl,combine whipping cream,sugar and vanilla. Beat until stiff peaks form. Frost sides and top of cake. Garnish with fresh strawberries.
Store in refrigerator.