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 instructions, typically 2-3 minutes. Pour batter into prepared pan and bake for 28-32 minutes until a toothpick inserted in the center comes out clean.
03 - Remove cake from oven and let 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 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 or spoon 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 in a pattern if desired for a marbled effect. Use all the gelatin liquid.
06 - Cover the pan with plastic wrap or foil and 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 powdered sugar and vanilla extract using an electric mixer or whisk until stiff peaks form, about 3-5 minutes.
08 - Spread whipped cream evenly over the chilled cake. Decorate with pastel sprinkles or jelly beans as desired. Slice into squares and serve cold.