01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan with cooking spray or butter.
02 - Combine vanilla cake mix, water, vegetable oil, and eggs in a large bowl. Beat according to package directions, about 2 minutes. 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 in the 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 stir 1 cup cold water into each bowl.
05 - Carefully pour strawberry gelatin over one half of the cake and lemon gelatin over the other half, or alternate pouring for a marbled effect. Ensure all the gelatin mixture is absorbed into the poked holes.
06 - Cover the pan with plastic wrap or aluminum foil. Refrigerate for at least 2 hours, or until gelatin is completely set.
07 - In a chilled bowl, beat heavy whipping cream with an electric mixer until soft peaks form. Add powdered sugar and vanilla extract, then continue beating until stiff peaks form and cream holds its shape.
08 - Spread the whipped cream evenly over the chilled cake. Decorate with pastel sprinkles or jelly beans as desired. Refrigerate until ready to serve.