Preheat oven to 350 degrees Fahrenheit. Spray a 9x13-inch baking dish with nonstick cooking spray.
Rinse strawberries and dry with a paper towel. Remove stems. Place strawberries in a blender or food processor. Pulse until crushed or blended smooth (desired consistency). Reserve 1 cup. Set aside.
In a large bowl, whisk the cake mix with strawberry gelatin powder (reserving 1 tsp of the strawberry gelatin powder for the frosting, if desired).
Add the eggs, oil, water, and 1 cup of crushed/pureed strawberries. Mix well.
Pour batter into the prepared baking dish. Bake at 350 degrees Fahrenheit for 30 to 33 minutes or until toothpick comes out clean when placed in the center of the cake. Allow cake to cool completely.
MAKE THE FROSTING: In a large bowl with an electric hand mixer, beat the butter until smooth and creamy. Add the strawberry preserves and 1 cup of powdered sugar. Blend well. Add 1 more cup of powdered sugar and the reserved teaspoon of strawberry gelatin (optional). Blend well. Spread over cooled cake. Serve with additional sliced fresh strawberries if desired.