Step 1

Preheat the oven to 350 degrees. Spray 2 (8-inch) round cake pans with cooking spray. Line the bottoms of the pans with parchment, spray the parchment, and set pans aside.

Step 2

Place berries in a large zip-top plastic bag and use a rolling pin to mash them. Set aside.

Step 3

Combine the cake mix, vegetable oil, milk, and gelatin in a mixing bowl and beat with an electric mixer until smooth. Beat in the egg whites 1 at a time, then beat in the vanilla. Beat in the strawberries and their juices until combined.

Step 4

Divide batter evenly between the prepared pans and bake for 30 to 35 minutes, until a toothpick inserted into the center of the cakes comes out clean. Set pans on a wire rack to cool completely before filling and frosting.

Step 5

Puree the berries in a blender or food processor. Pour into a small saucepan and add the lemon juice. Place pan over medium heat and bring mixture to a boil, stirring frequently. Turn heat to low and simmer, stirring often, until puree reduces by about half (8 to 10 minutes). You'll have about 1/3 cup of puree. Remove from heat and cool to room temperature.

Step 6

Once puree is cool, place the cream cheese and butter in the bowl of a stand mixer fitted with the paddle attachment. (You can also use a hand mixer and a large mixing bowl.)

Step 7

Beat until very smooth, then add 4 cups powdered sugar, 1 tablespoon milk, and salt and mix to combine. Beat in the puree until combined. Scrape down the sides and bottom of the bowl, then turn mixer to medium and beat until very smooth. Add additional powdered sugar if frosting is too thin and additional milk if frosting is too thick.

Step 8

Set one cooled cake layer on a cake stand or cardboard cake circle. Spread or pipe a layer of frosting on top. Top with a second cake layer, then frost and decorate the cake as desired. Slice and serve.