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 28-32 minutes until a toothpick inserted in the center emerges 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 approximately 1 inch apart and pressing about three-quarters of the way through the cake.
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. Allow to cool slightly.
05 - Carefully pour strawberry gelatin over one half of the cake and lemon gelatin over the other half, or alternate colors in sections for a marbled effect. Ensure all the liquid penetrates the poked holes.
06 - Cover the pan with plastic wrap or foil and refrigerate for at least 2 hours, or until the gelatin is completely set and the cake is thoroughly chilled.
07 - In a chilled bowl, whip the heavy cream with an electric mixer or whisk until soft peaks form. Add powdered sugar and vanilla extract, then continue whipping until stiff peaks form and the cream holds its shape.
08 - Spread the whipped cream evenly over the chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Slice into squares and serve cold, keeping refrigerated until ready to serve.