Spray 2 8-inch round cake pans with cooking spray. Line the bottoms with parchment paper and spray the paper with additional cooking spray. Set pans aside.

Mix and bake the cake according to directions on the box. Cool cake layers completely on a wire rack.

Once cake layers are cool, combine the water, strawberry gelatin, granulated sugar, and the 2 1/2 cups of berries in a medium saucepan set over medium heat. Cook mixture, stirring occasionally, until mixture comes to a rolling boil. Boil for 1 minute. Remove from the heat and cool for 10 minutes.

Poke 25 to 30 holes in each of the cake layers with a large-pronged fork or wooden skewer—don’t go all the way through the cake. Pour half of the gelatin mixture over each cake layer. Cover cake pans loosely with plastic wrap and refrigerate for at least 4 hours, or overnight.

When you’re ready to frost your cake, place the heavy cream in a large mixing bowl and beat with an electric mixer set to low speed. When heavy cream has thickened a bit, gradually add the powdered sugar and vanilla. Continue beating on low speed until mixture holds soft peaks, then turn mixer to medium and beat until it holds stiff peaks.

Carefully remove one of the cake layers from the pan and set strawberry-side up on a cake stand. (Run a knife around the edge of the cake to loosen; if you’re afraid the cake will break in half when you remove it, freeze the layers for 20 to 30 minutes.)

Spread about 1 cup of the heavy cream into an even layer on top of the cake. Top with the remaining cake layer, strawberry-side down. Frost the cake with the remaining whipped cream and garnish with remaining 1/2 cup of sliced berries. Serve and enjoy.