Prepare the vanilla cake mix batter, and bake as a 9x13-inch cake in a glass pyrex pan, using the instructions on the back of the box.
Remove the cake from the oven and let cool for 10 minutes. Then use the handle of a wooden spoon (or a wooden skewer) to poke holes all over the top of the cake.
In a large bowl, stir together the powdered jello with the hot water for 2 minutes, or until dissolved. It's important that it's dissolved. Then you can stir in the cold water until well-combined. Pour the jello mixture evenly over the cake. Chill for at least one hour.
Before serving, spread the Cool Whip evenly over the top of the cake. Store it in the refrigerator until serving time. Serve individual slices topped with fresh raspberries. Garnish each slice with a mint leaf.