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, vegetable oil, and eggs. Pour batter into prepared pan and bake for 28-32 minutes until a toothpick inserted in center comes out clean.
03 - Let cake cool for 15 minutes. Using a wooden spoon handle or skewer, poke holes evenly across the entire surface, spacing about 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 gelatins over separate halves of the cake, creating alternating colors for a marbled effect. Ensure all liquid is absorbed into the poked holes.
06 - Cover cake with plastic wrap and refrigerate for at least 2 hours until gelatin is completely set.
07 - Whip heavy cream with powdered sugar and vanilla extract using electric mixer or whisk until stiff peaks form.
08 - Spread whipped cream evenly over chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Slice and serve cold.