01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly.
02 - Prepare vanilla cake batter according to package directions using water, oil, and eggs. Pour 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 entire cake about 1 inch apart.
04 - In two separate bowls, dissolve each gelatin flavor in 1 cup boiling water. Stir until completely dissolved, then add 1 cup cold water to each bowl.
05 - Carefully pour or spoon gelatins into separate halves of the cake, alternating colors for marbled effect if desired. Use all liquid.
06 - Cover and refrigerate cake for at least 2 hours, allowing gelatin to set completely.
07 - Whip heavy cream with powdered sugar and vanilla extract until stiff peaks form.
08 - Spread whipped cream evenly over chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Slice and serve chilled.