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 mixing bowl. Beat according to package directions until smooth, approximately 2 minutes. Pour batter evenly into the prepared pan.
03 - Bake for 28 to 32 minutes or until a wooden toothpick inserted in the center comes out clean. Remove from oven and let cool in the 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 approximately 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 or spoon the strawberry gelatin over one half of the cake and the lemon gelatin over the other half, allowing liquid to seep into the poked holes. Alternate colors for a marbled pattern if desired.
07 - Cover the pan with plastic wrap or foil and refrigerate for at least 2 hours or until the gelatin is completely set.
08 - Using an electric mixer or whisk, beat the chilled heavy whipping cream, powdered sugar, and vanilla extract on high speed until stiff peaks form, approximately 3 to 4 minutes.
09 - Spread the whipped cream evenly over the chilled cake surface. Decorate with pastel sprinkles or jelly beans as desired. Slice and serve cold.