01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with cooking spray or butter.
02 - Combine vanilla cake mix with water, vegetable oil, and eggs. Beat according to package directions until smooth. Pour batter evenly into prepared pan.
03 - Bake for 28-32 minutes until a toothpick inserted in the center comes out clean. Remove from oven and let cool in pan for 15 minutes.
04 - Using the handle of a wooden spoon or a skewer, poke holes evenly across the entire surface of the warm cake, spacing them about 1 inch apart.
05 - 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.
06 - 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.
07 - Cover the pan with plastic wrap or aluminum foil. Refrigerate for at least 2 hours or until gelatin is completely set.
08 - In a chilled bowl, whip the heavy cream with powdered sugar and vanilla extract using an electric mixer or whisk until stiff peaks form.
09 - Spread the whipped cream evenly over the chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Slice and serve cold.