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 until smooth. Pour batter into prepared pan and bake for 28-32 minutes, or until a toothpick inserted into the center comes out clean.
03 - Let cake cool in the pan for 15 minutes. Using the handle of a wooden spoon or a skewer, poke holes evenly across the entire surface, spacing them approximately 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 colors for a marbled effect. Ensure all liquid is absorbed into the poked holes.
06 - Cover the pan with plastic wrap or aluminum foil and refrigerate for at least 2 hours, or until gelatin is completely set.
07 - In a chilled bowl, whip the heavy cream with an electric mixer until soft peaks form. Gradually add powdered sugar and vanilla extract, continuing to beat until stiff peaks form.
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.