Preheat oven to 350℉ and grease a 9x13 baking pan with nonstick cooking spray. Set aside.
Mix cake mix, melted butter, vanilla, and egg together in a mixing bowl. The dough will be stiff but continue to mix until it comes together. Press evenly into prepared pan with your fingers or the back of a spoon. Bake for 12-14 minutes or until set.
15.25 oz White Cake Mix, 6 tbsp butter, 1 egg, 2 tsp vanilla extract
Remove from oven and sprinkle with marshmallows. Return to oven and bake for another 2 minutes or until marshmallows are puffy. Remove from oven and set aside.
2 cup mini marshmallows
In a saucepan, mix chocolate chips and corn syrup. On medium low heat, constantly mix until melted and smooth. Remove from heat and stir in vanilla. Stir in cereal.
1 ½ cup chocolate chips, ⅔ cup light corn syrup
Place dollops of chocolate and cereal mixture onto the top of the marshmallows. Gently press down with the back of a spoon. Sprinkle with M&M candies.
2 cup rice cereal, 1 ½ cup caramel M&M candies
Let set for about an hour before cutting into 24 bars.