Ingredients:
 
1 package French vanilla cake mix (regular size)
1 cup buttermilk
1/3 cup canola oil
4 eggs
1 package (3 ounces) berry blue gelatin
1-1/2 cups boiling water, divided
1 cup cold water, divided
Ice cubes
1 package (3 ounces) strawberry gelatin
2/3 cup finely chopped fresh strawberries
1/4 cup fresh blueberries
3/4 cup butter, softened
2 cups confectioners' sugar
1 tablespoon 2% milk
1 jar (7 ounces) marshmallow creme
 
Directions:

Line a 13-in. x 9-in. baking pan with waxed paper.Grease the paper and set aside.
 
Combine the first four ingredients in a large bowl and beat on low speed for 30 seconds to start combining the liquid ingredients into the dry ingredients. Once they start to combine, beat the batter on medium for 2 minutes. Pour into the prepared pan once everything is thoroughly mixed and bake at 350 degrees for 35-40 minutes or until a toothpick inserted near the center comes out clean.
 
Cool for 10 minutes before removing the cake from the pan and placing it on a wire rack to cool completely.

Once the cake is completely cooled, transfer it to a covered cake board. Cut out a 5-in. x 4-in. rectangle (1/2 in. deep) with a small knife in the top left corner of cake, leaving a 1/2-in. border along edges of cake.
 
For the red stripes, cut out 1/2-in. wide rows (1/2 in. deep), leaving a 1/2-in. border. Use a fork to carefully remove the cut out cake pieces.
 
Dissolve the berry blue gelatin in a small bowl with 3/4 of a cup of boiling water in it. Pour a 1/2 cup of cold water into a 2-cup measuring cup and add enough ice cubes to measure 1-1/4 cups. Stir into gelatin until slightly thickened. Scoop out and discard any remaining ice cubes. Use this same process to make the strawberry gelatin.

Combine the strawberries and 1 cup strawberry gelatin in a small bowl. In another bowl, combine the blueberries and 1 cup blue gelatin.
 
Refrigerate both of them for 20 minutes or just until soft-set. Save the remaining gelatin for another use.
 
Stir the gelatin mixtures and slowly pour the blueberry mixture into rectangle and spoon the strawberry mixture into stripes.
 
Beat butter in a large bowl until it is fluffy then beat in the confectioners' sugar and milk until smooth. Add the marshmallow creme and beat the mixture well until it is light and fluffy once more. Spread 1 cup over the sides and the top edge of cake. Refrigerate the remaining frosting for 20 minutes.

Cut a small hole in the corner of pastry or plastic bag and insert a large star tip.
 
Fill the bag with remaining frosting and pipe the frosting in between the rows of the strawberry gelatin and around the edges of the cake.
 
Refrigerate for 1-2 hours minimum so the gelatin can set, but overnight is best.