Preheat oven to 375 degrees F. Spread frozen strawberries on a parchment-lined baking sheet. Roast 20 to 25 minutes, stirring once, until soft and syrupy. Cool slightly, then puree. Reserve 1/2 cup for the batter and set the rest aside for filling.
Reduce oven to 350 degrees F. Grease and flour two 9-inch round cake pans or line with parchment. In a large bowl, combine cake mix, egg whites, oil, buttermilk, sour cream, vanilla, and 1/2 cup cooled strawberry puree. Beat 2 to 3 minutes until smooth and fluffy. Divide between pans and bake 25 to 30 minutes until a toothpick comes out clean. Cool in pans 10 minutes, then transfer to wire racks to cool completely.
In a chilled bowl, beat heavy cream, powdered sugar, vanilla, and cornstarch on medium-high until stiff peaks form. Refrigerate until ready to use.
Place one cake layer on a serving plate. Spread remaining strawberry puree over the top. Scatter chopped fresh strawberries evenly over the puree.
Place second cake layer on top. Frost top and sides with whipped cream frosting.
Arrange thinly sliced strawberries in a decorative ring along the bottom edge. Add additional garnish on top if desired. Chill at least 1 hour before slicing.