01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with cooking spray or butter.
02 - Prepare vanilla cake batter according to package directions using water, oil, and eggs. Pour batter into prepared pan and bake for 28-32 minutes, or until a toothpick inserted in the center comes out clean.
03 - Allow cake to cool for 15 minutes. Using a wooden spoon handle or skewer, poke holes evenly across the entire cake surface, 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 or spoon each gelatin flavor over separate halves of the cake, alternating colors to create a marbled effect if desired. Ensure all liquid is absorbed into the holes.
06 - Cover cake tightly with plastic wrap or foil. Refrigerate for at least 2 hours to allow gelatin to set completely.
07 - Using an electric mixer or whisk, beat heavy whipping cream with powdered sugar and vanilla extract until stiff peaks form.
08 - Spread whipped cream evenly over the chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Slice and serve cold.