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. Pour batter into prepared pan and bake for 28-32 minutes until a toothpick inserted in the center comes out clean.
03 - Allow cake to cool for 15 minutes. Using the handle of a wooden spoon or a skewer, poke holes evenly across the entire surface of the cake, spacing them approximately 1 inch apart.
04 - In two separate bowls, combine strawberry gelatin with 1 cup boiling water and lemon gelatin with 1 cup boiling water. Stir each until completely dissolved, then add 1 cup cold water to each bowl.
05 - Carefully pour the strawberry gelatin over one half of the cake and lemon gelatin over the other half. If marbling is desired, alternate pouring small amounts of each flavor across the entire surface. Ensure all holes absorb the liquid.
06 - Cover the pan with plastic wrap or foil and refrigerate for at least 2 hours, or until the gelatin is completely set and the cake is thoroughly chilled.
07 - Using an electric mixer or whisk, beat the chilled heavy whipping cream, powdered sugar, and vanilla extract until stiff peaks form. Do not overbeat.
08 - Spread the whipped cream evenly over the chilled cake. Decorate with pastel sprinkles or jelly beans as desired. Slice into squares and serve cold.