To make the cake:Heat oven to 350 degrees. Grease and flour three 9-inch cake pans and set aside. In a large mixing bowl, beat sugar and butter at medium speed with an electric mixer until fluffy. Add strawberry extract, puréed strawberries, and sour cream. Beat until combined.
In a medium mixing bowl, sift together flour, baking powder, soda, and salt. Gradually add to butter mixture, beating until combined.
In a clean, medium bowl, beat egg whites until stiff peaks form. Gently fold into batter. Pour batter evenly into prepared pans.
Bake for 35-40 minutes or until a wooden toothpick inserted in the center comes out clean. Let cool in pans for 10 minutes then turn out onto wire racks. Cool completely.
To make the frosting: In a large bowl, beat butter and cream cheese with an electric mixer at medium speed until smooth. Add puréed strawberries and strawberry extract beating until combined. Gradually beat in confectioners’ sugar until smooth.
Spread frosting between cake layers and on the top and sides. Refrigerate until ready to serve. Garnish with fresh sliced strawberries, homemade vanilla ice cream, strawberry sauce and a sprig of mint, if desired.