01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with cooking spray or butter.
02 - Prepare the vanilla cake batter according to package instructions using water, oil, and eggs. Pour into the prepared pan and bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean.
03 - Let the cake cool for 15 minutes. Using a wooden spoon handle or skewer, poke holes evenly across the entire cake, spacing them 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 the gelatins into separate halves of the cake, alternating colors for a marbled effect if desired. Use all the liquid.
06 - Cover and refrigerate the cake for at least 2 hours, allowing the gelatin to set completely.
07 - Whip the heavy cream with powdered sugar and vanilla extract until stiff peaks form.
08 - Spread the whipped cream evenly over the chilled cake. Decorate with pastel sprinkles or jelly beans if desired. Slice and serve chilled.