01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with cooking spray or butter.
02 - Combine vanilla cake mix, water, vegetable oil, and eggs in a large bowl. Beat according to package directions until smooth, approximately 2 minutes. Pour batter into prepared pan.
03 - Bake for 28 to 32 minutes or until a wooden toothpick inserted in center comes out clean. Remove from oven and let cool in pan for 15 minutes.
04 - Using the handle of a wooden spoon or a skewer, poke holes evenly across entire cake surface, spacing about 1 inch apart and reaching nearly to bottom of cake.
05 - In two separate bowls, dissolve strawberry gelatin in 1 cup boiling water and lemon gelatin in 1 cup boiling water. Stir each until completely dissolved, then add 1 cup cold water to each bowl.
06 - Carefully spoon or pour strawberry gelatin over one half of cake and lemon gelatin over other half. For marbled effect, alternate colors in rows. Gently press surface to ensure gelatin seeps into holes.
07 - Cover pan with plastic wrap or foil. Refrigerate for at least 2 hours until gelatin is completely set and cake is thoroughly chilled.
08 - In chilled bowl, beat heavy whipping cream with electric mixer on high speed until soft peaks form. Add powdered sugar and vanilla extract. Continue beating until stiff peaks form.
09 - Spread whipped cream evenly over surface of chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Refrigerate until ready to serve.
10 - Cut into squares and serve cold. Store any leftovers covered in refrigerator for up to 3 days.