Step 1: Preheat the oven to 350°F. Grease a 13"x9" baking pan with cooking spray and prepare the cake according to the package directions.
Step 2: Bake the cake until a toothpick inserted into the center comes out clean, about 20 to 25 minutes. Allow it to cool.
Step 3: Using a bamboo skewer or chopstick, poke holes all over the cake, spacing them 1/2" to 1" apart.
Step 4: In a small saucepan, bring 1/2 cup of water to a boil. Add the Jell-O powder and whisk until dissolved. Remove from heat, whisk in 1/2 cup of cold water, and pour the gelatin mixture evenly over the holes in the cake. Cover and refrigerate for at least 2 hours or overnight.