01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly.
02 - Combine vanilla cake mix with water, vegetable oil, and eggs. Mix according to package directions. Pour batter into prepared pan and bake for 28-32 minutes until a toothpick inserted in center comes out clean.
03 - Let cake cool for 15 minutes. Using a wooden spoon handle or skewer, poke holes evenly across entire surface, spacing approximately 1 inch apart.
04 - In 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.
05 - Carefully pour or spoon gelatin mixtures onto separate halves of cake, alternating colors for marbled effect if desired. Ensure all liquid is absorbed into poked holes.
06 - Cover cake and refrigerate for at least 2 hours to allow gelatin to set completely.
07 - Whip heavy cream with powdered sugar and vanilla extract until stiff peaks form.
08 - Spread whipped cream evenly over chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Slice and serve chilled.