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 2-3 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 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 the strawberry gelatin over one half of the cake and lemon gelatin over the other half, allowing liquid to seep into the poked holes. Alternate colors for a marbled effect if desired. Use all the gelatin liquid.
06 - Cover pan with plastic wrap or foil and refrigerate for at least 2 hours, or until gelatin is completely set throughout the cake.
07 - In a chilled bowl, whip heavy cream with powdered sugar and vanilla extract using an electric mixer or whisk until stiff peaks form.
08 - Spread whipped cream evenly over the chilled cake surface. Decorate with pastel sprinkles or jelly beans as desired. Serve chilled.