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, typically 2 minutes. Pour batter into prepared pan and bake for 28-32 minutes until a toothpick inserted in center comes out clean.
03 - Allow cake to cool in pan 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 about 1 inch apart.
04 - 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.
05 - Carefully pour strawberry gelatin over one half of the cake and lemon gelatin over the other half, or alternate spoonfuls for a marbled effect. Ensure all poked holes absorb the liquid.
06 - Cover the pan with plastic wrap or aluminum foil. Refrigerate for at least 2 hours or until gelatin is completely set and cake is thoroughly chilled.
07 - In a chilled bowl, whip heavy cream with an electric mixer or whisk until soft peaks form. Add powdered sugar and vanilla extract. Continue whipping until stiff peaks form and cream holds its shape.
08 - Spread whipped cream evenly over the chilled cake. Decorate immediately with pastel sprinkles or jelly beans in festive patterns. Refrigerate until ready to serve.