Prepare the white cake mix according to package directions. Pour into a greased 9x13 inch pan and bake at 350F until a toothpick comes out clean, 25-30 minutes. Let cool completely on a wire rack, about 1 hour.

Puree 1.5 cups strawberries with sugar until smooth. In a small bowl, sprinkle gelatin over water and let stand 5 minutes until softened. Microwave gelatin mixture 20-30 seconds until dissolved, then whisk into strawberry puree.

Use the handle of a wooden spoon to poke holes all over the cooled cake, about 1 inch apart and deep enough to reach the bottom. Slowly pour strawberry mixture over cake, letting it seep into holes. Reserve remaining 0.5 cup chopped strawberries for garnish. Refrigerate 1 hour to set.

Beat cold heavy cream, powdered sugar, and vanilla on medium-high speed until stiff peaks form, 3-4 minutes. Spread evenly over chilled cake. Top with sliced strawberries and reserved chopped berries. Refrigerate until ready to serve, at least 30 minutes.