01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan with cooking spray or butter.
02 - Combine vanilla cake mix with water, vegetable oil, and eggs in a large bowl. Beat according to package directions until smooth, approximately 2 minutes.
03 - Pour batter into prepared pan. Bake for 28-32 minutes until golden brown and a toothpick inserted in center emerges clean.
04 - Allow cake to cool in pan for 15 minutes. Using handle of wooden spoon or skewer, poke holes evenly across surface at 1-inch intervals, pressing almost to bottom.
05 - In separate bowls, combine strawberry gelatin with 1 cup boiling water, stirring until fully dissolved. Repeat with lemon gelatin in second bowl. Add 1 cup cold water to each bowl.
06 - Carefully pour strawberry gelatin over left half of cake and lemon gelatin over right half. Gently tilt pan to distribute liquid evenly into holes.
07 - Cover pan with plastic wrap and refrigerate for minimum 2 hours or until gelatin is completely firm.
08 - Using electric mixer or whisk, beat chilled heavy cream, powdered sugar, and vanilla extract until stiff peaks form, approximately 3-5 minutes.
09 - Spread whipped cream evenly over chilled cake surface. Sprinkle with pastel sprinkles or arrange jelly beans decoratively across top.
10 - Cut into 12 rectangular pieces. Serve cold for best texture and flavor presentation.